General Considerations of Fungi | Botany

In this article we will discuss about the general consideration of fungi.

Discharge and Dissemination of Spores in Fungi:

The actual process of spore discharge or spore liberation in fungi is the detachment of spores from the spore-bearing structure, and spore dissemination is the subsequent movement of the spores before coming to rest on a substrate.

Spore discharge may be an active or a passive process. Actively discharged spores are projected a greater or lesser distance by a variety of devices generally based upon the hydrostatic pressure of the spore-bearing structures or upon devices triggered off through changes in turgidity or the development of negative pressures in cells adjacent to the spores or in the spores themselves.

Whereas, amongst the passive spore discharge are number of different methods depending on whether the spores are dry or slimy, enclosed by a membrane or associated with material which attracts other organisms to it.

Spore discharge exhibits a surprising number of periodic patterns. Many of these are related to the diurnal rhythm of alternating day and night. Others are related to the water relations of the reproductive structures, and yet others reflect changes in climatic cycle.

Temperature and rainfall, the two external factors which greatly effect the development of reproductive structures, themselves appear to be the prime determinants in long term periodic fluctuations of spore discharge. Temperature, humidity and light are often determinants of diurnal rhythms.

It has been observed that the day-maximum periodicity of Erysiphe spores is relates to light, that of Fomes annosus basidiospores to temperature.

Spore dissemination again is controlled by various physical agencies of which most important is wind. The velocity of wind is again an important factor for distributing the spores in distant areas from the source of spore development (Fig. 307).

I. Active Spore Discharge in Fungi:

Spores may be violently discharged by the mechanisms like:

(i) The bursting of spore-producing structures;

(ii) Sudden chan­ges in shape of turgid spores or of turgid structures associated with the spores;

(iii) Rapid twisting movements produced as a result of drying in filamentous sporangiophores or by hygroscopic movement;

(iv) Sudden breaking of tensile water in conidia or conidiophores, distorted on drying, which are thereby permitted to return to their original form; and (a) by impaction. The distance of projection of spore discharge depends on the initial velocity of the projective and on its size, shape and density.

In violently discharged fungal spores and spore masses there is a general positive correla­tion between size and distance of shooting. The principal external conditions effecting violently spore discharge are temperature, light, water supply and humidity of air.

(i) Bursting of spore-producing structures:

At maturity or soon thereafter, the spores of many fungi are forcibly discharged.

Some of the commonly encountered devices are:

(a) Trigger Devices:

The observations of Meredith (1961 —63) have shown that in some cases, dry spores are liberted by an active discharge mechanism. The cell concerned loses water by evaporation so that it develops a negative pressure and tends to be distorted. As the process continues, water molecules in the cells tend to cohere both amongst themselves and the walls.

Thus the water is exposed to an increasing tension until a point is reached where the tension is released by the water expanding into a gas phase and so the distorted walls recover.

This rapid process imparts a jerk, at recovery from tension, discharges one or more conidia into the air; in others, how­ever, gas bubbles develop in one or more of the conidia immediately prior to discharge, e.g., Curvularia, Alternaria. In all cases the jerk of recovery is the affective agency of liberation but in some cases is, perhaps, assisted by hygroscopic movements of the conidiophores.

The most striking discharge mechanism due to increased turgor pressure of cells is known by the Gasteromycete, Sphaerobolus. Here the basidiocarp is a minute sphere with peridium enclosing a spherical glebal mass—peridiole, embedded in a gela­tinous matrix.

The peridium comprises of six histologically distinct layers. When mature, the peridium splits to give six to eight teeth and, at the same time, a split develops between the outer three and the inner layers except at the apices of the teeth.

The innermost layer gelatinizes so that the peridiole rests in a drop of fluid held in an inner and an outer cup. The innermost layer of the inner cup is composed of large parenchyma-like cells which, initially, are full of glycogen.

This becomes hydrolysed to glucose, hence its suction pressure increases and it takes up water. The cells tend to expand but the outer layer of the inner cup is composed of rigid, tangentially running inter-woven fibres which resist expansion. The tension so set up is finally released by a sudden inversion of the inner cup, accompanied by a minute detonation, which thereby projects the peridiole (Fig. 308A to G).

(b) Syringe Devices:

The essential process involved in this type of spore libera­tion is the development of a high hvdrostatic pressure within a cell. This type of spore liberation is found particularly in Pilobolus. In Pilobolus the walls of the apices of the sporangiophores are extremely elastic and have a circular transverse line of weak­ness around their apices (Fig. 186).

The hydrostatic pressur of the sporangiophore increases until it ruptures, then it contracts violently, and squirts out a jet of water and so projects the sporangium. Particular interest applies to Pilobolus because of its phototropic response which results in the tip of the sporangium being directed towards the maximum light intensity (Fig. 308F to J).

The mechanism of syringe discharge is better understood in the Ascomycetes. In the Erysiphales, the asci within the cleistothecium take up water and swell. The cleistothecium is ruptured and the ascus or asci within protrude or may even squeeze out and finally be violently ejected.

Continued swelling of the contained or ejected asci results in their bursting and scattering the enclosed ascospores in all directions (Fig. 308D and E).

In the Discomycetes the exposed hymenium of the apothecium permits the simul­taneous discharge of ascospores and this gives rise to the phenomenon of ‘puffing’, i.e., synchronized discharge of ascospores from mature asci (Fig. 309A). As ascus becomes turgid, a positive hydrostatic pressure often associated with the development of a vacuole forces much of the cytoplasm and the ascospores to the apex of the ascus.

The ascus frequently enlarges and may protrude from the rest of the hymenium in a state of high tension as in Ascobolus. This tension is released when the operculum gives way and the elastic walls of the ascus contract, driving out the upper half of the contents as a cylindrical jet which dissipates into a cloud of ascospores carrying droplets. The distance of discharge may be even tip to 17 cm.

In inoperculate Discomycetes asci dehisce by an irregular apical split, e.g., Geoglos- sum, Ttichoglossum. In these genera the ascospores are filiform and septate and after dehiscence an ascospore is pushed through the apical pore, slowly at first but with increasing velocity until the last spore has been ejected; the ascus then collapses.

Not all inoperculate Discomycetes behave like this, however, and some are capable of effective ‘puffing’, e.g., Helotium, Sclerotinia and Rhytisma acerinum.

Pyrenomycetes never exhibit ‘puffing’ for the ostiole of the perithecium, enclosing the asci, is usually so narrow that only one ascus can pass through it at a time. There are three types of active discharge of spores in the Pyrenomycetes. The first is illus­trated by Sordaria Jimicola.

The asci remain attached to the base of the perithecium. They elongate up the neck canal, one at a time, until the tip is forced through the ostiole (Fig. 309B). Increasing internal hydrostatic pressure leads to the rupture of the ascus and the spores are forced out singly or in groups (Fig. 309B and C).

A second type is seen in Sporomia intermedia. The ascus wall is double layered, the outer ectoascus being thick and rigid, the inner endoascus thinner and extensible. As the ascus imbibes water it tends to swell but this is resisted by the ectoascus until it is finally ruptured by the internal pressures.

The endoascus then elongates suddenly to two to three times its original length and penetrates the ostiole. The ectoascus is ruptured, its upper part separating as a thimble-shaped lid (Fig. 309F, G), the endoascus elongates pushing through the ostiole and the spores are discharged. Finally the empty ascus contracts and disintegrates (Fig. 309D to G). The third type is found in Ceratocystis.

The asci become detached and lie free within the perithecium. They pack the perithe- cial cavity and are forced into the neck. Finally one protrudes, it swells but its lower half is firmly held by the ostiole so that it bursts apically, liberating the ascospores, and is itself extruded when empty, its place being taken at the ostiole by the next ascus (Fig. 309H and I).

In some other Pyrenomycetes the asci collect in a mucoid drop and are disseminated by water.

The active discharge of basidiospores of Hymenomycetes is associated with the secretion of a small drop of liquid and the spores are discharged by drop-excretion mechanism (Fig. 259). One basidiospore only is projected at a time but all four follow each other successively, usually in opposite pairs (Fig. 310A to F).

Once the basi­diospores have been launched into the air they are free to be dispersed by air current (Fig. 307). After discharge there is no detectable shrinkage of the sterigmata. A simi­lar condition has been observed by Buller and his associates in the members of the Tilletiaceae (Fig. 310G to I) and Uredinales.

(ii) Sudden changes in shape of turgid spores or of turgid structures associated with spores:

Spore discharge is often associated with sudden rounding off of turgid cells of fungi. In Entomophthora coronata a single and relatively large conidium is borne at the end of a straight conidiophore-the tip of which projects into the conidium as a distinct columella. As the conidium tends to round off, the re-entrant part bulges outward and, as a result, the conidium bounces off to a distance (Fig. 310J to L).

A similar mechanism is encountered in the genus Sclerospora. Here the area of contact between the sporangium and its sterigma is relatively small and the tip of the latter does not project into the sporangium; there is merely a flat contact, rounding off in this region brings about discharge of sporangium (Fig. 311A & B). In most rusts (Uredinales) the aeciospores are violently discharged within the aecium.

The spores are polyhedral because of mutual pressure. There is a tendency for them to round off when conditions are sufficiently damp. This happens suddenly and spores bounce out either singly or in groups to a distance of several millimetres. In the Erysiphales, conidia are actively discharged by rounding off.

(iii) Spore liberation by hygroscopic movements:

This mode of liberation occurs in dry spored fungi. In Peronospora tabacina, the dichotomously branched sporangiophore twists dislodging the sporangia through hygroscopic movements consequent upon drying out (Fig. 311G to F). In some Gasteromycetes, spore liberation is effected by hygroscopic movement of the capillitial threads.

(iv) Sudden breaking of tensile water in conidia or conidiophores:

It has been shown recently that, in a number of dematiaceous fungi, conidia are set free by the breaking of tensile water and release strains. Meredith in 1961 observed that Deightoniella torulosa the end cell of the conidiophore, which bears a single conidium, has a thickened wall. Because the thickening is uneven, evaporation leads to a temporary distortion so that the upper region of the end cell is drawn downward.

However, this cell, because of the rigidity of its wall, tends to return to its original form, and the fluid contends are under increasing tension as evaporation proceeds. Finally, a break occurs either because the cohesion between the water molecules or their adhesion to the cell wall is overcome.

When this happens a minute gas bubble makes its appear­ance and instantaneously enlarges as the distorted cell returns to its original shape with a sudden jerk which throws off the attached conidium to a distance of several centimetres (Fig. 311G to K).

(v) Spore liberation by impaction:

The best known examples of this type of spore liberation are the puff-balls and their alies (Lycoperdales), and the various types of ‘splash-cups’ such as those of the bird’s nest fungi (Nidulariales). In the Lycoperdales the contents of the basidiocarp are a mass of dry spores interspersed with fine dried-out hyphae, the capillitial ihreads.

The basidiocarp opens by a simple (Lycoperdon) or complex (Geastrum) ostiole. Spores may be blown out by the wind across the ostiole.

They may also be forced out when the basidiocarp is struck by raindrops, when air is expelled with spores; the peridium then springs back to shape (Fig 312G to J). Brodie (1951) has studied splash-cup dispersal mechanism in the Nidulariales (Fig. 312A to D).

For details see general account of the series Gasteromycetes. The impact of large drops of water from a height on a twig covered in conidial stroma ta of Nectria causes the drops to break into several droplets containing conidia. These droplets are scattered at a great distance.

II. Passive Spore Liberation:

In a large number of fungi the spores are not actively discharged. In them, spore liberation depends on wind, rain splash, or the activity of insects and other organisms. Many fungi have dry spores which, although not violently discharged are fairly easily set free by sufficiently strong air currents.

As to the fungi producing slime spores, these spores cannot be blown off. Spore liberation by rain splash or insect dispersal of these fungi seems to be the rule. The mechanism of passive spore liberation in principle, however, is not very different from the process of spore dissemination.

(i) Liberation of Dry Spores:

Some of the most important epidemic producing pathogenic fungi such as the rust fungi (Uredinales), smut fungi (Ustilaginales), and the downy and powdery mildews (Peronosporales and Erysiphales) have dry spores. These spores are detached and dispersed by the wind. This is also true of many of the common molds (Penicillium) and (Aspergillus).

Dry-spored types are most effectively liberated at low relative humidities and high Wind-speeds. Separation devices include various modifications like gelatinization of the wall between the spore and hypha, intercalary cells or disjunctors as in Albugo and aeciospores of many rusts.

The pres­sure exerted by the intercalation of disjunctor leads to a rupture of the membrane in the plane of the septum and the sporangia or spores as the case may be are sepa­rated by the disjunctor. In some cases small hygroscopic movement of the conidio­phore or sporangiophore is sufficient to liberate conidia or spores from sporangia. Subsequently, splashing of raindrops may also help dispersal of conidia or spores.

(ii) Liberation of Slime Spores:

In many of these, the spores are liberated as a consequence of the gelatinization of their walls or through the autolysis of adjacent Structures or cells. Good examples are found in the Mucoraceae where the sporangium wall dissolves, leaving the spores held together in a drop of mucilage containing water, e.g., Mucor sp.

A similar kind of behaviour is exhibited by many conidia producing fungi where the conidia accumulate to form slimy drops at the ends of the conidio­phores, e.g., Verticillium sp.

These drops are not blown away by the wind and, in a dry atmosphere, the spores or conidia involved are left sticking firmly to each other. While sticky they may be liberated through contact with a passing animal, or by spreading out like oil over a film of water. When dry they may become detached as a consequence of some physical shock.

In many Pyrenomycetes spore formation is associated with the production of mycilaginous material produced by autolysis of sterile cells of the perithecium as in Chaetomium or pycnidium in Diatrype. In all such cases the spores, embedded in slime, are exuded through the ostiole.

A rather different form of spore liberation is achieved in those cases where the spore-drop or associated slime is attractive to animals, especially insects. The spores become detached in the attractive material which is then removed by the animal. Examples are the sugar-containing droplets exuded by the spermogonia of many rusts, the oidia of agarics and the Sphacelia stage of Claviceps purpurea.

The best known examples of these are the stinkhorns of various types (Phallus, Dictyophora, Mutinus) and the hollow, spherical lattice-wall of Clathrus. Spore liberation is here dependent upon a complex series of processes. The basidia in all cases are embedded in a dark-green sugary gluten which encloses the hymenial surfaces and is itself encased in a gela­tinous material and a peridium, this being spherical or egg-shaped.

The ‘eggs’ are subterranean and evidently take up water so that the volva swells. When this happens the characteristic sticky-sweet odour which is attractive to flies develops. The stalk or network, as the case may be, now enlarges, the peridium splits and the hymenium, with its covering of gluten, is carried above the ground where the material, including the spores, is rapidly removed by flies (Fig. 294F).

III. Spore Dissemination:

Spore dissemination is essentially a passive process determined by physical agencies such as (i) wind, (ii) water, (iii) rain splash, (iv) living organisms, and (v) other means. It is not often clearly distinguishable from some of the processes like, spore liberation by wind, rain splash or insects.

(i) Dissemination by wind:

Spores once discharged are either below or above the boundary layer of turbulent air. In the former case the air is still and the spores fall to the ground. In the latter, however, they are air borne in the turbulent air of the atmosphere and become dispersed by air currents (Fig. 307). Spores which are projected violently through the air follow a quasi-parabolic curve like most other projectiles.

(ii) Dissemination by water:

Ingold and others have studied the dissemina­tion of spores of aquatic fungi. According to them, the dissemination is dependent upon concentrations of spores and water speeds and stimuli of chemical substance that may remain in water.

(iii) Dissemination by rain splash:

A-classical example of this process is exhi­bited by the members of the Nidulariales, details of which have been observed by Brodie. The process has been described in the general account of the series Gasteromycetes.

(iv) Dissemination by living organisms:

Dissemination by living organisms is very common. The living organisms responsible for spore dispersal are: insects and other animals.

(a) Dissemination by insects:

The Gasteromycetes, particularly the members of the Phallales are a group with particular adaptations to this mode of dissemination. Another common kind of attractor is the production of nectar-like secretation as in Clavi­ceps purpurea (conidial stage), the pycnial stage of many rust fungi and, in some cases, their aecial stage. The insects visit the fungus primarily to feed on the attractive material such as nectar.

During feeding the insects’ body gets smeared with conidia or spores as the case may be. When the feeding is over, the insects naturally carry conidia or spores over to whatever substrate they touch. Thereby carry with them conidia or spores and indirectly help dissemination of conidia or spores. The spores of Dutch elm disease pathogen, Ceratocystis ulmi are dispersed by bark beetles, Scolytus spp.

(b) Dissemination by other animals:

Certain animals, such as dogs and pigs are attracted by the odour of fruit body of truffles (Tuber spp.) which they dig out from the ground and feed on and the spores pass through their alimentary canal and pass out through fecal matter at distant places and thus the spores are disseminated.

The spores of a number of coprophilous fungi, notably species of Pilbolus, Ascobolus, Sordaria, and Coprinus are dispersed by horses, cattle, sheep, goats, etc. These animals swallow the spores and herbage together, the spores with chewed herbage pass through alimentary tract and are ultimately voided in the feces.

By the time the feces are discharged from the alimentary tract, the animals might have travelled quite a distance thereby the spores are disseminated in distant places.

(v) Dissemination by other means:

Fungal spores are often disseminated through spore-infected seeds, soil, manure, agricultural implements and often human hands and feet. Besides these, human beings quite often unknowingly disperse fungal spores through nursery stocks and similar other media.

(B) Heterokaryosis:

Blakeslee (1906) first reported the germination of zygotes of the Mucoraceae obtained under controlled conditions. He obtained a number of ‘neutral’ mycelia from the germination of zygotes of Phycomyces blakesleeanus. These mycelia grew abnormally, tended to produce zygophore-like branches (pseudophores) and were bright yellow as a consequence of the abundant carotene in them.

It was supposed that such mycelia carried nuclei of different mating types, + and -, in their hyphae. Burgeff (1912, 1914) found the same phenomenon in the same species. He removed the tip of a hypha of + mating type and inserted it into the cut of a hypha of — mating type, so that cytoplasm and nuclei intermingled.

The resultant mycelium, derived from this ‘myxochimera’, grew in a manner, similar to those of Blakeslee’s ‘neutral’ cultures. Burgeff described such mycelia as heterokaryotic.

The phenomenon of the co­existence of genetically different nuclei heterokaryons in the same cytoplasm of the same individual is known as heterokaryosis.

Heterokaryosis, as a natural pheno­menon, is virtually confined to the fungi. Its significance for the biology of the fungi was not appreciated until pathologis’s like Brierley (1929), realized that it could account for some of the natural variation shown by phytopathogenic fungi.

Hansen and Smith (1932) showed that if single spores of Botrytis cinerea were isolated and cultured, they gave rise to colonies with various attributes which could be grouped into three main types, a, b and x.

Types a and b were different but constant in their cultural and morphological characteristics. Type x, however, was inconsistent. It gave rise to sectors to types a and b in addition to areas sharing its characteristic, irregular mor­phology; this type of segregation was also shown by conidia isolated from type x cultures.

They obtained strains of types a and b that were constant for several subcultures and then grew them together. As a consequence of hyphal fusions which they saw and in which they detected nuclei, presumably migrating, they reconstituted a mycelium of type x. In 1935, Kohfer carried out a similar analysis of Mucor mucedo and obtained by selection homokaryotic mycelia from the original heterokaryotic strain.

In 1938, Hansen reported similar studies on 35 genera of Fungi Imperfecti, in which 32 showed this phenomenon; it, therefore, seemed to be widely distributed. The assumption was made that two distinct types of nuclei can co-exist in B. cinerea hyphae and that when spores which are multinucleate are abstricted, differing proportions of these nuclei may be incorporated in them.

Sometimes by chance all nuclei will be the same (types a and b), but more usually they will be different (type x). The products of germination will be correspondingly stable or unstable. Jinks (1959) offered another explanation for it. Since the origin of the nuclei which entered the spores was not determined; this is the somatic segregation of different cytoplasmic determinants (plasmons) from a heteroplasmon.

Such segregations have been demonstrated by Jinks and his co-workers and by others.

Heterokaryosis may arise in any of the following ways:

(i) By the germination of a heterokaryotic spore producing a heterokaryotic somatic condition.

(ii) By the fusion of vegetative cells of genetically distinct nuclei and cytoplasm which intermingle in the fusion cells, produce hyphae bearing heterokaryons resulting from multiplication of cells and repeated division of heterokaryotic nuclei.

(iii) By mutation.

(iv) By fusion of nuclei of same genetic constitution and the subsequent multi­plication of the diploid nuclei so formed resulting in a mixture of haploid and diploid nuclei. No new genetic combination would be formed in such heterokaryosis.

Developmental details of Heterokaryosis: 

The development of Heterokaryons or the phenomenon of Heterokaryosis proceeds along following lines:

1. Hyphal Fusion:

The frequency with which hyphal fusion occurs is subject both to genetics and to environmental control. The ability of a heterokaryon to retain a well-mixed nuclear population, and thus phenotype uniformity, depends in part on fusion among the hyphae near the frontier and the number of nuclei per cell.

Any statical tendency toward segregation of nuclear types during branching of hyphae will be reversed to a greater or less extent by fusions.

However, hyphal fusion is only effective when it is followed by nuclear migration at least into the fusion cell.

2. Nuclear Migration:

Buller (1931) first described this phenomenon in Coprinus spp. using nuclei of different mating types. He showed that, in compatible matings, nuclei can apparently migrate from a small mycelium into a large one. The presence of both nuclei is newly formed cells, could readily be detected by the presence of clamp connections.

Since then the phenomenon has been studied in several other Homobasidiomycetes and in Ascobolus stercorarius. Migration proceeds at remarkably rapid rates. It may or may not be affected by light. The determination of which myce­lium is to be the donar and, which the acceptor may be genetic or environmental.

The actual mechanism of nuclear migration is not known. It is often assumed that the nuclei are carried passively by cytoplasmic streaming.

3. Cytoplasmic Streaming:

The growth of filamentous fungi is correlated with cytoplasmic streaming; this will tend to randomize the nuclear population if the nuclei move freely with the stream. Even if the nuclei are not carried easily with the cytoplasm, the movement of cytoplasm will facilitate phenotypic, homogeneity over large regions.

4. Nuclear Division:

Nuclear division is, of course, the ultimate focus of selec­tion, and aside from differences among nuclei in division rate, changes in nuclear ratios must reflect differences in division rates among regions of different nuclear constitution. In many fungi particularly the Ascomycetes, nuclear division takes place rapidly in hyphal tips and for a greater or lesser distance behind them.

In fact, division is reported to be simultaneous. The timing of nuclear division is determined by the common cytoplasmic condition.

Genetic Control of Heterokaryosis:

In Neurospora heterokaryosis is determined by two pairs of alleles, G/c and D/d. Only mycelia carrying like alleles will form heterokaryon, i.e., CD and CD, Cd and Cd, cD and cD or cd and cd but not CD with Cd or cd and so on. When hyphae of unlike alleles are brought together, hyphal fusions are not prevented but the fusion cell is rapidly sealed off.

In a short time bubble-like structures appear and the cell dies of autolysis. It has been called ‘Heterokaryon incompatibility’, is found in Aspergillus nidulans and other species of Aspergillus.

Significance of Heterokaryosis:

While the demonstration of heterokaryosis may be credited to Burgeff (1912, 1914) the significance of heterokaryosis in nature was first fully appreciated by Hansen and Smith (1932), working with isolates of Botrytis cintrea.

Stable heterokaryons are with a selective advantage, usually in rate of growth over their components. These may arise in case of heterokaryons containing nuclei with nutritional defects as a result of complementation, i.e., the deficient of one nucleus is made good by the ability of the other and vice versa. In general, this effect must operate in the cytoplasm.

By heterokaryosis heterotrophic organisms can survive in nature by creating balanced nutritional conditions among two or more nutritionally deficient strains. Such condition is brought about by-the fusion of cells without any nuclear intermixing.

It has been demonstrated that strain of Aspergillus niger incapable of producing conidia although the mycelium grows almost as well as the wild type and is appa­rently normal when the nuclei are combined experimentally in a diploid nucleus, a mycelium containing such nuclei grows and produces conidia normally.

In this case, therefore, complementation, as a physiological process, can only occur at the intra­nuclear level, not in the cytoplasm. Similar effects may also be seen in relation to dominance and the quantitative expression of dominant genes can be studied by changing heterokaryons.

Many of these interactions are manifested in what Dodge (1942) called ‘hetero­karyotic vigour’ and this has been compared with heterosis in diploid organisms. Since this heterokaryotic vigour to a greater »or less degree may be expected to be the rule in nature it will not be surprising to find heterokaryosis in most natural populations of fungi.

It is clear that the heterokaryotic situation is usually explicable in terms of complementation.

Nevertheless, complex situations do arise involving non-alletic genes and their suppressors and little study has yet been made of such complications. Attention should also be drawn to the fact that recessive genes, including lethals, can arise in heterokaryosis and persist until they segregate in spores.

The major initial work of complementation was that of Dodge (1942) with Neuro- spora tetrasperma, followed by that of Beadle and Goonradt (1944) with N. crassa. Dodge found that two slow-growing mutants were able to complement one another in hetero­karyotic association to yield a rapidly growing mycelium.

In fact, heterokaryons have been used widely in tests of allelism, as well as in the study of complex loci, alleles of which may complement to some extent through inter­action of mutant proteins. Since a wild type (normal) nucleus may mark the defi­ciencies of a mutant nucleus and as such a heterokaryon will resemble the wild type.

If two nuclei carrying non-allelic mutations are combined in a heterokaryon, the result­ing phenotype will approach the wild-type condition. The latter behaviour is desig­nated “complementation”.

Heterokaryosis is likely to be of some importance in nature for it represents an ideal system ensuring rapid and successive adaptations of a fungus to a continually hanging environment. This is a situation that most fungi must constantly be in through their own activities, both catabolic and secretory.

Moreover, experimental studies with heterokaryons have shown that it is exceedingly difficult to eradicate entirely any particular type.

Such nuclei, which may be of little or no adaptive significance in one situation, may be of enormous significance in another. It is for this reason that more experiments on selection with heterokaryons are desirable so that both the possibilities or, and rates of change of nuclear ratios can be fully assessed.

That selection can be efficacious and that new properties can be shown by hetero­karyons compared with those of their parental strains, has been in semi-natural situa­tions. For example, Puccinia graminis tritici produces a race virulent to a wheat, hitherto only resistant to one of the parental strains of the heterokaryons.

Buxton (1954, 1956) also showed the virulent parental strains could be combined to produce heterokaryons which were quite a virulent as naturally occurring pathogenetic strains.

(C) The Parasexual Cycle:

Up to the middle of the present century the normal sexual cycle as encountered in the Phycomycetes, Ascomyctetes and Basidiomycctes, was considered the only mecha­nism of genetic combination in fungi. Lack of knowledge of recombination processes was a serious drawback in certain areas of study in the Deuteromycetes (Fungi Imper­fecti) in which normal sexual cycle has not been observed.

They are: control of viru­lence, breeding of improved industrial strains, the study of genetic diversity within related groups, etc.

There was no suitable answer to the question “how did they meet the challenge of evolutionary forces?” With the discovery of processes other than normal sexual cycle, often known as “alternatives to sex” has opened up a new line of genetic approach of recombination mechanisms in fungi and related organisms.

One of these novel processes of recombination, was discovered and elucidated in Aspergillus nidulans, the imperfect stage of Emericella nidulans and designated by Pontecorvo and Roper (1952)—-the parasexual cycle.

The parasexual cycle, as defined by Ponte­corvo, is the spontaneous occurrence in hyphae heterokaryosis, fusion of genetically dissimilar nuclei, and recombination and segregation of the diploid nuclei so formed to the development of haploid ones.

An obvious prerequisite for recombination is fusion of haploid nuclei. This normally requires, as a first step, inclusion of the genetically different nuclei in the same cytoplasm. This is achieved through heterokaryosis. There are several ways in which heterokaryosis is established.

The most common way perhaps is by the fusion between cells of genetically different somatic hyphae. Due to nuclear migration, cytoplasmic streaming and division of newly migrated nuclei the hyphae become hetero- karyotic.

The heterokaryotic condition may be established by mutation in one or more nuclei and another way is by the fusion of some of the haploid nuclei of the same hypha and their subsequent division and spread among the haploid nuclei. Since the last method of heterokaryosis involves no new genetic combination, this type of hetero­karyosis is not of much significance.

In a heterokaryotic mycelium, nuclear fusion takes place between haploid nuclei of different genotypes as well as of the same genetic constitution resulting in hetero­zygous and homozygous diploid nuclei respectively. As such a heterokaryotic myce­lium may contain as many as five types of nuclei.

They are: two types of haploid, two types of homozygous diploid, and heterozygous diploid nuclei; all of them multiply at about the same rate.

But initially the diploid nuclei are much smaller in numbers than the haploid ones. The way in which nuclear fusion occurs is a mystery. It might be possible that in a crowded hyphal apex with 20 to 30 nuclei, nuclei divide synchronously. The overlapping divisions become intermingled and a new nuclear membrane encloses two haploid sets of chromosomes.

They have been actually demons­trated and designated as ‘streaming-mitoses’. The frequency of such nuclear fusion is usually low.

The diploid nuclei divide mitotically and increase in number. During mitotic division crossing over occasionally takes place resulting in new combinations known as mitotic recombination. This is probably the most important phase in the para­sexual cycle. The recombination’s so formed give the fungus some of the advantages of sexuality without possessing it at all.

But overall, the recombination’s are entirely dependent on the existence of heterokaryosis. Indeed the recombination’s obtained by parasexual mechanism are by far much smaller in number than those achieved through sexual mechanism. This mechanism is, however, very significant in the Deuteromycetes where sexual mechanism is not known.

The next stage is the sorting out of haploid and diploid nuclei. This takes place by the development of haploid and diploid conidia which produce haploid and diploid mycelia respectively. This may take place in nature and also can be isolated by arti­ficial means. J. A. Roper (1952) first isolated diploid strains of Aspergillus nidulans.

It has also been observed that a diploid colony of mycelium may have areas where haploid conidia are produced. This indicates that the diploid nuclei undergo haploidization in the mycelium and are sorted out in course of mycelial growth. The frequency of spontaneous development of somatic diploids is usually low and has been traced in A. nidulans, A. niger, Penicillium chrysogenum, and in Coprinus lagopus.

In Ustilago maydis, however, diploids have been obtained as a consequence of failure of normal meiosis. Mitotic recombination is also a rare process. There is also suggestive evidence for a parasexual cycle in Puccinia graminis tritici. The significance of the two modes of recombination, somatic and meiotic, in the same fungus is problematical.

The sequence of events that follow in a parasexual cycle may be outlined as: heterokaryosis, fusion between nuclei (like or unlike), multiplication of both haploid and diploid nuclei with occasional mitotic crossing over of diploid nuclei, sorting out of diploid nuclei with occasional haplodization of diploid nuclei, and sorting out of haploid nuclei.

It is now quite certain that pansexuality occurs in several Deuteromycetes which lack a normal sexual cycle. In them, parasexuality is responsible for genera­ting variability in the mycelium though its mechanism is far from being adequately understood.

Parasexuality has also been detec­ted in the Ascomycetes and Basidiomycetes with a normal sexual cycle as well. Special mention may be made of: Cochliobolus sativus, Ustilago maydis, and Schizophyllum commune. It was Bradley who in 1962 for the first time demonstrated the presence of parasexual cycle in fungi possessing normal sexual cycle.

(D) Mycorrhiza:

The name ‘Mycorrhiza’ in its broadest sense is the non-pathogenic or feebly pathogenic association of fungi and the roots or rhizomes of flowering plants, thalli or other parts of non-flowering plants. Both the organisms may derive benefit from the association, or the fungus becomes pathogenic when the other component is weak.

The components’ are associated in a nutritional, also designated as mycotrophic relationship with each other.

But strictly speaking, the term mycorrhiza is used only where the organ concerned is a root. Roots provide an important habitat for these special ecological groups of fungi. The presence of fungus may be essential for good growth of the host plant, assisting in mineral uptake by its roots.

The host plant pro­duces carbonaceous compounds, vitamins, etc., which it supplies to the fungus and the fungus absorbs the nutrients from the soil and provides the host plant with these. In this way both components develop.

Associations of fungi with the thalli, rhizomes, or other parts of plants should be reasonably termed ‘mycothalli’ and ‘mycorrhizo- mata’, though these are not widely accepted. One of the main distinguishing features of mycorrhizal organs is that the plants which possess them do not suffer acute disease due to the presence of fungal hyphae in them.

Indeed, mycorrhizal organs are very often the main absorbing organs of many plants which form few uninfected roots under natural conditions.

The knowledge of the existence of mycorrhizas dates back to the fourth century B.C. The mycorrhizal associations appear to have occurred in fossil club mosses (Lepidodendron spp.), of the early Devonian period. They are characterized by several infection points and the development of some coils of intracellular hyphae.

But defi­nite information with regard to their true structural nature started with the classical work of Frank in 1885.

Frank introduced the name mycorrhiza to the composite fungus-root organs of the Cupuliferae. He showed that seeds of certain Orchids germinate only when in association with right type of fungi. Frank also pointed out that in some forest trees, mycorrhizas are formed with a number of different fungi, but in others the association is specific.

But Burges, A (1936) established that the mycor­rhizal association is an example of limited parasitic attack having no symbiotic value.

The physiological and biological significance of mycorrhiza was not thoroughly understood for long time and therefore caused some disagreement among botanists in interpreting the behaviour of its components. Some botanists regarded mycorrhizal fungi as parasites upon the roots.

Whereas others believed the relationship between the fungi and roots in a mycorrhizal association is more complex than fungal parasi­tism and that both fungi and roots benefit from their close association in mycorrhiza. According to this latter view, the fungi receive food from root tissues, and in turn facilitate certain physiological activities of roots.

Through experimental trials it has been established that mycorrhizal roots have greater absorption rate and disease immunity than nonmycorrhizal ones. It is also true, the fungal component behaves as a parasite as soon as the associated non-fungal component becomes weak.

Occurrence of Mycorrhiza:

Mycorrhizal infection is not universal. Trees on soils rich in minerals nutrients may be quite uninfected. Aerable land, well manured soil or disturbed soil are less favourable for mycorrhizal infection. The occurrence of mycorrhiza is very common in poor soil, particularly in the forest trees.

Mycorrhizal components:

The components of Mycorrhiza are as follows.

(i) Nonfungal:

Mycorrhizas are common among the angiosperms (members of the families—Gupuliferae, Orchidaceae, Ericaceae, Monotropaceae, Leguminosae, Genteniaceae, Gramineae, and Palmae); among the gymnosperms (members of the families—Pinaceae and Taxaceae); among the pteridbphytes (species of the genera Lycopodium and Ophioglossum); and among the bryophytes (species of the genera— Marchantia, Anthoceros, Pellia, Anuria, Lobelia and Lolium).

(ii) Fungal:

The fungal components of mycorrhiza are: among the basidiomy­cetes (species of the genera Russula, Lactarius, Clitocybe, Tricholoma, Amanita, Armillaria, Marasmius, Boletus, Rhizopogon and Scleroderma); among the ascomycetes (species of the genera Tuber and Elaphomyces); among the phycomycetes (species of the genera Pythium and Endogone); and among the deutoromycetes (species of the form-genera Phizoctonia and Coenococcum).

There are wide variations in the behaviour and distribution of the mycorrhizal fungi.

Some (Amanita muscaria and Boletus edulis) exhibit wide host range; again, Boletus elegans forms mycorrhiza with Larix species only; again others may behave differently in different situations, e.g., Armellaria mellea in Japan grows as a pathogen on conifers, whilst at the same time associated with the mycorrhizal tubers of the Orchid Gastroidea elata.

Even a single fungus species (Acaulospora laevis) has been reported in Australia, Brazil, England, Scotland, New Zealand, South Africa, the U.S.A. and Pakistan. It forms mycorrhiza in almost all families of flowering plants including gymnosperms (except the family Pinaceae), as well as in many ferns and liverworts.

Mycorrhizal development:

Seedlings remain uninfected until active photo­synthesis begins. In the early stages of growth when the tap root and long roots are uninfected, the short roots become infected from the soil. The fungal hyphae enter into rootlets either through root hairs or rhizoids or through outer layer by direct penetration.

They may grow intercellularly or intracellularly remaining restricted in the cortical cells, do not affect stelar tissues and chlorophyllous tissue. Mycorrhizal development is dependent upon the growth cycle of the host, season, and habitat variations, arising from human activity,

Fungal behaviour:

The haphae may form external sheath on the root surface or loose weft of hyphae may remain externally or connect with soil merely by a few fungal strands. Intercellular hyphae produce specialized organs arbuscules (haustoria) or vesicles or both arbuscules and vesicles and many other specialized structures—sporangioles, peloton, etc. in the host cells.

Nature of Mycorrhizal root:

Due to mycorrhizal infection, the pattern of development of the parent root is changed. Those rootlets which become infected remain active and branch repeatedly growing slowly in length by dichotomous branch system. Hence there is increase in short root production and thereby absorption sur­face of root system is increased.

This is commonly encountered in humus soil. The mycorrhizal roots may also become coralloid and ball-like in shape. The best known mycorrhizas are those of forest trees and Orchids: these represent the two main types of fungus-root association—the ectotrophic and the endotrophic.

I. Ectotrophic mycorrhiza:

In the ectotrophic mycorrhizas many of the fine lateral rootlets are enveloped in a mantie or sheath of light pseudoparenchymatous fungal tissue which prevents growth in length of the root causing it to assume a swollen or coraxloid form.

The light pseudoparenchymatous fungal tissue sends branches inward between the cortical cells of the root and outward in the soil and isolates the root from the soil and comprises a considerable part of the total weight and volume of the whole organ.

The hyphae which penetrate the cortex form a network, often called the Hartig-net, in the middle lamella of the cell walls and the intercellular spaces of the external cortex. Hyphae pass outward into the soil and ramify among the soil particles. These may be scarce or abundant. In some species an internal Hartig-net only is present in the cortex, in others only an external sheath is found.

The main mother roots by which the’ root system spreads are not usually completely infected by the mycorrhizal fungus. As new lateral roots are produced from the mother roots, they become mycorrhizal during their passage through the cortex or sheath so that a permanent state of mycorrhizal infection persists after initial infection in the seedling stage.

Forest trees produce large number of fine roots which are differen­tiated into long and short roots. Short roots are feeding roots as their main function is absorption. Shortest roots become mycorrhizal. In them, hairs are absent and caps are poorly developed. They develop many branches. Clowes (1949) showed that the difference in anatomy between mycorrhizal and non-mycorrhiza roots of species of Fagus is in a cell shape.

In infected roots some cortical cells shew reduced longitudi­nal extension and increased transverse expansion and, in addition, meristematic activity might be reduced. These changes account adequately for the swollen shape of the mycorrhizal roots. Slankis (1958) showed that Boletus tariegatus induces increase in the frequency of short-root laterals having dichotomous forking in species of Pinus.

This suggests the morphogenetic effect of the fungus on the roots. Ectotrophic mycorrhiza is common in many of the forest trees belonging to the genera: Pinus, Picea, Cedrus, Abies, Fagus, Quercus.

Among the basidiomycetes, species of Russula, Lactarius, Clitocybe, Tricholoma, Amanita, Boletus, Scleroderma and Rhizopogon; and among the ascomycetes species of Tuber and Elaphomyces are involved in ectotrophic mycorr­hiza formation. Coenococcum graniforme, a deuteromycetous fungus is also involved with mycorrhizal association.

Some of these fungi exhibit wide host range, e.g., Amanita muscaria, Boletus edulis; others show host specificity, e.g., the genus Pinus has its selected group of “pine” mycorrhizal fungi and Boletus elegans forms mycorrhiza only with larch.

The basidiomycetous fungi of ectotrophic mycorrhiza are physiologically specialized in their dependence upon simple carbohydrates for growth. Most of them utilize glucose and fructose, some also mannose and sucrose. Others can hydrolyze starch, but few are able to utilize cellulose.

Mycorrhizal trees have greater vigour. The soil mycelium attached to the mycorrhiza can tap a greater volume of soil than the host root. All nutrients entering the host root first pass through the fungal hyphae.

The interdependence of host and fungus is well understood in cases where the fungus produces extrahyphal enzymes which break down lignin and hydrolyze cellulose of humus and plant debris making suitable for the utilization of the forest plants, e.g., Boletus subtomentosus with Pinus montana and Lactarius deliciosus with Pinus sylvestris.

The Ectotrophic Mycorrhizas are basically coralloid and ball types. According to Melin and others, on the basis of external forms, the mycorrhizas may be differen­tiated into:

(i) Short dichotomously branched roots invested with mantles of various colours, the colour being determined by the species of fungus involved in its produc­tion, it is very common in nature;

(ii) Knots or tuber-like growths developed by the fusion of mantles produced by merged clusters of forked roots; and

(iii) Long, thin unbranched structures which occur upon the roots.

II. Endotrophic Mycorrhiza:

This is the commonest type of mycorrhiza virtually ubiquitous being present in tropical, temperate and arctic regions. It is found in almost all families of flowering plants including gymnosperms (absent in the Pinaceae), as well as in many ferns and liverworts. Endotrophic mycorrhiza is also characterized by the presence in plants of economic importance such as grasses, cereals and legumes.

Here the external hyphal mantle or sheath is lacking, but present in Monotropa and among the members of the family Ericaceae. Again among the members of the Orchidaceae the external mycelium is scanty. In general, the fungal component forms externally on the root surface a loose weft of hyphae without ensheathing it. The branches of these hyphae enter the root cortex and penetrate cortical cells.

Fungal hyphae which may be septate or non-septate, on gaining entrance in the root through root epidermis, very rarely through root hairs, grow inter- or intracellularly. They do not exploit green tissues, nor do they attack storage organs such as tubers. The endotrophic mycorrhizal fungi are not destructively parasitic as they occur at certain times as part of the normal process of root development.

Roots become dichotomously branched when they become mycorrhizal increasing surface area of absorption. It has also been found that endotrophic mycorrhizas are associated in the nutrient absorption of saprophytic flowering plant Monotropa.

The fungi also influence the germination of seeds and spores and the early development of the young and adult plant by providing food material and enzymes essential for metabolism. Besides this, they also break down cellulose and lignin in the soil and supply carbon compound to their nonfungal components.

The main types of endotrophic mycorrhiza are distinguished as follows:

(i) That caused by fungi with septate hyphae, usually basidiomycetous (Armillaria mellea) or deuteromycetous fungi with basiodiomycetous affinity (species of Rhizoctonia which have produced fruit bodies of Corticium, Tulasnella, Ceratobasidium). The nonfungal component belongs mainly to the Ericales, Orchidaceae, Gentianaceae; and few pteridophytes and bryophytes.

(ii) That caused by aseptate hyphae of phycomycetous fungi (species of the genera Pythium and Endogone, and Rhizophagus-Endogone complex); and the nonfungal component belongs to the families Gramineae, Palmae, Legumi- nosae, Compositae; in gymnosperms (other than Pinaceae); pte-idophytes and bryophytes.

In some forest trees, mycorrhizas are formed with a number of different fungi, but in others the association is specific, e.g., strains of Rhizoctonia-like fungus with certain orchids; and Armillaria mellea with a Japanese orchid Gastroidea elata.

Depending on the nature of fungal and nonfungal components, fungal hyphae behave in the following manner:

(a) The hyphae of larger diameter grow intercellulary producing intercalary or apical large and thick-walled swellings—the vesicles. The vesicles contain oil and are believed to function as storage organs or resting spores when the roots decay. Such mycorrhiza is known as vesicular mycorrhiza (VM).

(b) Branches of main hyphal system usually of smaller diameter pass into cells and produce arbuscules (haustoria) which are often dichotomously branched. The apices of the arbuscules become swollen to form “sporangioles”. The sporangioles may become free within the host cells, but disintegrate there.

This is known as arbuscule-sporangiole complex. The whole arbuscule-sporangiole complex may disintegrate in the host cells. This is common in the Leguminosae Papilionaceae) and Gramineae.

(c) The hyphae may also produce combination of vesicle and arbuscule and thus known as—vesicular-arbuscular mycorrhiza (VAM). It is also known as phycomycetous mycorrhiza. It is found wide-spread in the plant kingdom,

(d) Again form-species of Rhizoctonia produce intracellular hyphae in the roots of Orchids which become coiled as pelotons to be digested in the root cell.

Fossilized vesicular-arbuscular mycorrhizas have been found in the Carboni­ferous and Devonian strata. Their modern counterparts have been investigated, especially in the Gramineae. Two types have been identified, although both were formerly included in the genus Rhizophagus: they are species of Pythium and certain other fungi which are possibly small forms of Endogone.

Significance of Mycorrhiza:

Convincing proof of the functional nature of mycorrhiza, however, is lacking. Among bryophytes internal mycelium has been found of common occurrence in certain genera. But whether this relation is mutualistic still lacks positive evidence. Similar is the condition with the pteridophytes. Nume­rous theories have been put forward as to the true nature of mycorrhizas.

But in general, there are two lines of thought:

(a) Mycorrhizas are patholpgical structures induced by the parasitic action of the fungus upon the root tissues; and

(b) That the mycorrhizas are symbiotic structures.

Mycorrhizal infection induces increase in dichotomous branching of roots followed by profuse growth of rootlets increasing surface area of absorption of roots. Thereby mycorrhizal plants receive better nutrition than nonmycorrhizal ones.

Again majority of the feeding roots of ectotrophic mycorrhizal host are covered by a layer of fungal sheath which helps higher absorption rate. Besides this, the soil mycelium attached to the mycorrhiza can tap a greater volume of soil than the host root. All nutrients entering the host root first pass through the fungal hyphae.

As such the mycorrhizal fungal hyphae play a very important role in nutrient absorption. Also this shows that the host is dependent upon the fungal partner for nutrients absor­bed from the soil.

Mycorrhizas are especially important for plant nutrition in deficient soils where they may greatly increase the availability of phosphorus and several other nutrients not readily accessible to the plant. Many reports show that mycorrhizas enhance the uptake of nitrogen, phosphorus, Calcium, Na, Fe, Cu, Bo, Zn, Al, and Strontium; but reduce the uptake of K, and Mn.

Since adequate phosphorus nutrition is required for root nodule formation, mycorrhiza may be a precondition for nodulation under deficient soil condition (Brown 1968, 73; Sanders and others 1975). Besides this, mycorrhizas remain active on old roots for relatively long period of time increasing absorption rate.

It has been widely established that mycorrhizal plants when growing on nutrient deficient soil produce greater dry weight and absorb disproportionately larger amounts of nutrients from the soil than uninfected plants.

Hence mycorrhizas are especially important for plant nutrition in deficient soil due to increase in the total and relative absorbing area of the root system and increased ability to accumulate nutrients per unit area for which mycorrhizal trees have greater vigour.

Slankis (1973) showed that auxin production enhanced during the dichotomous branching of roots induced by ectotrophic mycorrhizas helps in the elongation of radial walls of their cortical cells.

Mycorrhizal association releases inhibitory compounds to establish barriers against infection by pathogenic fungi. Thereby the host plant does not suffer from any acute disease.

The fungal partner gets suitable habitat for growth on the root surface where essen­tial requirements are provided by the root itself during mycorrhizal association. Roots also excrete various organic substances which stimulate fungal spore germina­tion and encourage root penetration.

Again the mycorrhizal fungus influences the germination of seeds, the early development of young plant, or may stimulate the growth and activity of the adult.

It provides enzymes essential for metabolism of the partner. Mycorrhizal association is almost universal in Orchids as because the my­corrhizal fungi provide germinating Orchid seeds with sugars and sometimes Vitamins (thiamine and biotin) and amino acids (Smith 1974).

Form-species of Rhizoctonia form mycorrhizal association with terrestrial Orchids (Talbot 1966-67). This is also true with regard to the germination of Cinchona seeds.

The interdependence of host and fungus is well understood in cases where the fungus produces extrahyphal enzymes which break down lignin and hydrolyze cellu­lose of humus and plant debris making suitable for the utilization of the forest plants, e.g., Boletus subtomentosus with Pinus montana and Lactarius deliciosus with Pinus sylvestris.

The interdependence of host and fungus in mycorrhizal relationship is, therefore, seen to involve the supply of major and minor nutrients as well as morphogenic factors to both partners and demonstrates how closely integrated are the two components, fungus and host which form nutrient absorbing organs.

This behaviour of the host and fungus is better understood in cases where an exchange of enzymes and Vitamins (thiamine and biotin) and amino acids takes place. Hence, there is little doubt that mycorrhizal infection has a morphogenetic effect leading to prolonged life, growth, and branching of the rootlets.

Larger number of basidiomycetous fungi are mycorrhizal, they are dependent on simple carbon compounds which have been provided by their hosts.

Species of Endogone are totally dependent on association with plant roots for growth. In the initial stages of infection they require the stimulation of exudation from the host for the determination of the direction of growth of the hyphae, for the pro­duction of appressoria and for penetration of the host tissues.

An interesting phenomenon is encountered in the saprophytic flowering plant Monotropa which is associated with endotrophic mycorrhiza in which fungal compo­nent helps in nutrient absorption. Here carbonaceous food passes from neighbouring spruce and pine trees through the mycorrhiza] fungal hyphae to the cells of Monotropa.

Such a relationship is also found in some saprophytic Orchids, and is termed epiparasitism.

Mycorrhizas play important role in afforestation. It has been demonstrated that pine and spruce seedlings exhibit nitrogen stervation and eventually die unless they are invaded with mycorrhizal fungi. The seedlings of Pinus strobus do not grow well without mycorrhizal fungi. Of the mycorrhizal fungi experimented, Boletus viscidus produced best results.

By field trial it has also been established that some members of the Boletaceae form mycorrhizas with forest trees and may be important in the esta­blishment of pine plantation. Besides this, it has been farther established that the seedlings with mycorrhizal fungi are immtine to various diseases and pests.

Pseudomycorrhizas:

The term pseudomycorrhiza was first introduced by Melin to short roots attacked by certain root parasites and other fungi in the absence of mycorrhazal fungi, so that the cells were penetrated, growth was slow, and branching was rare or absent. The fungus particularly involved was Rhizoctonia sylvestris.

This weak pathogen does not compete successfully with mycorrhizal fungi for the coloniza­tion of the surface tissues of the roots in normal conditions. Usually the root becomes invested to some extent with a kind of sheath of fungal tissue and the cortex is per­meated by hyphae which penetrate the cells to form structures called haustoria.

When seedlings of pine and spruce are infected, some decrease in vigour of growth which varies with the species, occurs. R. sylvestris does not cause damage to the hosts. Actually pseudomyconhiza formation is stimulated in conditions where mycorrhizas fail to form.

(E) What Are Fungi?

The fungi are regarded as members of the Plant Kingdom and certainly, in general aspects, the majority of them bear a superficial resemblance to plants.

That the fungi do not really belong to either of the Plant and Animal Kingdoms, but constitute a third co-equal Kingdom, is often observed by mycologists. Follow­ing are some of the aspects of fungi that deserve special consideration to finally decide what the fungi are.

One fundamental difference from the plants is that none of tie fungi bear chloro­phyll and are not capable of effecting photosynthesis. Plants, in general, utilize simple substances and build them up into substances of greater complexity by various processes of which photosynthesis is of primary importance. True, fungi are incapable of synthesizing carbohydrate in a process similar to that of green plants.

But it has been experimentally established that large number of fung’ whose metobolic activities have been worked out, can synthesize complex organic substances in special processes of their own which can seldom be compared with those of plants and animals. Fungi are, however, similar to green plants in being fixed and possessing cell walls though they differ fundamentally in their lack of chlorophyll.

The fungi show great differences in size, structure, and metabolic activities. Some of the fungi are unicellular, while others, such as the mushrooms and other higher fungi form large fruit bodies of complicated structure with elaborate mechanism for propagation.

The faculty of fruit body development, an outstanding feature of large number of fungi, has virtually provided an ample scope to consider fungi to be different from plants and animals.

Again according to Langeron (1945), a further fundamental difference between plants and animals on the one hand and fungi on the other is that the latter never form tissue. All structures, including the highly organized fruit bodies of the higher fungi, consist entirely of a system of tubes.

Even when the tubes are apparently divided into individual cells by cross walls, these invariably have a central pore, through which both cytoplasm and nuclei can freely pass. Langeron maintains that fungi are thus essentially unicellular.

The cell wall material of the fungi is made up of chitin, which, though has resem­blance with that of animals is chemically quite different has already been established by Weitstein as early as 1910. The animal chitin is much more complex than the fungus chitin, the similarity being very superficial.

With gradual evolutionary progress in fungi the interval between plasmogamy and karyogamy increases and there is interpolation of a third phase, the dikaryophase, in between the haplophase and diplophase. This can very well be traced in the diff­erent groups of fungi.

In the Phycomycetes the life cycle comprises only of two phases, the haplophase and the diplophase, the latter being very short and the former is pre­dominant.

Whereas in the Ascomycetes, there is clear interpolation of dikaryophase in between the haplophase and diplophase, though the dikaryophase is entirely dependent on the haplophase in every respect. Finally, in the Basidiomycetes there is clear occur­rence of the three phases (haplophase, dikaryophase, and diplophase) which are entirely independent from each other.

The dikaryophase is rather predominant in the entire life cycle. The dikaryophase, an unique feature of fungi, is not encountered in any other living organism.

The fungi have not evolved from the algae by the loss of chlorophyll is now an accepted fact. Whereas, the fungi had a common ancestry with the protozoa but split off at a very early stage of organic evolution. If this line of evolution is found to be correct, the fungi do not belong to any of the two Kingdoms—Plant and Animal.