In this article we will discuss about the classification of charales.
Family Characeae:
The Characeae &re cosmopolitan, occurring in all continents and in many isolated oceanic islands. The plants are aquatic or subaquatic in habit with elaborately developed plant body having rhizoidal prostrate portion and aerial portion differentiated into nodes and internodes. The aerial portion is attached in the soil by hyaline rhizoids.
The cells are with numerous chloroplasts and becoming multinucleate at maturity. Vegetative parts consist of an uniaxial series of alternating short nodal and elongated inter-nodal cells.
The nodal cells produce peripheral nodal cells from which arise a whorl of branches or 1-2 branches. In addition, 1-celled processes may arise at branchlet nodes or from gametangial laterals.
Sexual reproduction is oogamous and involves fertilization by biflagellate antherozoid which fuses with the egg while it is held in the oogonium surrounded by a sterile jacket of spirally twisted cells (convoluting cells). Such an oogonium should preferably be termed as oocarp. The antherozoids arise singly from cells in antheridial filaments enclosed in a complex antheridiocarp (taxonomically called the antherodium).
The antherozoid gains access to the ovum through a terminal aperture produced between the convoluting cells. After fertilization the zygote develops into a thick-walled oospore. On germination of the oospore a horizontal filament (protonema) develops as a lateral branch. The normal, erect shoots arise from the protonemata.
Meiosis is known to occur in the first division of the zygote,’ prior to initiation of the protonema. Vegetative propagation occurs by secondary protonemata which may develop from axial nodes, by bulbils, or by fragmentation.
Genus Chara of Characeae:
It is a submerged aquatic alga growing attached by rhizoids to the muddy or sandy bottom of small pools, lakes , and slowly flowing streams. The plant body is slender and flexuous, with individual plants attaining lengths 20 to 30 cm. or more in height.
The multicellular plant resembling the appearance of a miniature horse-tail, has an erect main axis differentiated into a regular succession of nodes and internodes with whorls of lateral branches of limited growth, often spoken as ‘leaves’, arising from the nodes (Fig. 86A).
Branches of limited growth having nodes and internodes just like the main axis may arise axillary to the leaves. The internode is composed of a Central cylindrical cell having length many times longer than breadth ensheathed by elongated narrow cells arising from basal nodes constituting the cortex (Fig. 86D).
The corticated nature of the stem is clear in cross-section. Incrustations of calcium carbonate make the plants coarser justifying the common name stonewort.
The multicellular rhizoids with oblique septa developed from the lowest node are not differentiated into nodes and internodes. Their functions are fixation and absorption. The apical and nodal cells are uninucleate with dense cytoplasm having no conspicuous vacuole, bear discoid chloroplasts without pyrenoids.
Whereas, the elongated internodal cells are multinucleate with numerous small chloroplasts embedded in cytoplasmic layer lining a huge central vacuole. The inner layer of cytoplasm exhibits very conspicuous streaming movement.
The growth of the main axis and its branches is initiated by a single dome-shaped apical cell which cuts off segments parallel to its flat base (Fig. 86B). Each segment divides transversely into a biconcave upper nodal initial cell which divides and redivides to give rise to a node and a biconvex lower internodal initial cell which remains undivided but elongates considerably to form the internode (Fig. 86C).
Vegetative propagation usually occurs at all stages during the lifetime of the alga and is effected by the production of vegetative propagative structures which are:
(i) Star-shaped aggregated cells filled with starch that are developed about the lower nodes;
(ii) Tuber-like structures formed on the rhizoids; and
(iii) Protonema-like outgrowths originating from the rhizoids.
The alga exhibits a high degree of specialization in the nature of the gamete- producing structures which are named as globule and nucule Instead of being called as antheridium and oogonium respectively. With a few exception, most species of Char a are homothallic-having both globule and nucule always borne at the nodes with’ a definite orientation, the nucule always lying above the globule (Fig. 86D).
A mature globule is large, stalked, spherical in structure and bright yellow to red in colour (Fig. 87A).
Its wall is composed of eight curved plates, the shield cells, joined end to end giving the wall a pseudocellular appearance (Fig. 87B). Each shield cell bears in its centre a rod-shaped cell—the manubrium from whose distal end arise rounded cells—the primary capitula, each of which produces sue secondary capitula.
From each of these capitula arise two long, whip-like threads—the antheridial filaments (Fig. 87G & D). Each antheridial filament is composed of single row of cells ranging between 100 and 200 in number, each of which produces one elongate biflagellate antherozoid (Fig. 87E & F).
The nucule is a stalked oval structure being surrounded by an envelope of five long filaments placed side by side arranged spirally exhibiting a twisted appearance and affording protection to the enclosing oogonium. Each filament terminates in a small erect cell.
The five erect cells of the five filaments together form five-celled crown or corona capping a mature nucule (Fig. 88D & E). The oogonium produces a single large uninucleate ovum packed with starch grains and oil globules. The nucleus in the ovum lies at the base. The apex of the ovum remains clear and is occupied by finely granular cytoplasm which constitutes the receptive spot.
The development of the globule takes place from the apical cell of the lateral branch which cuts off one or two discoid cells at its base and then becomes spherical. As a result of longitudinal followed by transverse division, the spherical cell develops into octants, each of which undergoes further division and by enlargement, the inner segments become separated from one another.
The eight peripheral cells develop into eight shield cells and gradually from the centre of each shield cell there arises a manubrium from which in turn develop primary and secondary capitula and antheridial filaments.
The adaxial cell of the basal node becomes the nucule initial which divides twice giving rise to a row of three cells, the uppermost of which develops into the oogonium proper, the lowest forms the stalk, and the middle cell cuts off five peripheral cells which elongate into filamentous structures forming tube cells.
These tube cells surround the oogonium in a spiral fashion and finally each one cuts off coronal cell at the apex (Fig. 88A to G).
With maturity, the enveloping tube cells become dilated as a result of which there arises a narrow slit at the apex of the oogonium occupied by a mucilaginous liquid which permits easy entrance of antherozoids in the oogonium (Fig. 89A). The antherozoid nucleus traverses down in the base of the oogonium where the fusion of both the nuclei takes place.
Immediately after fusion, the zygote nucleus travels to the apex of the oospore surrounding which develops a thick wall. Later, the thick-walled oospore with other structures of the nucule drops to the bottom of the pool.
The surrounding envelope of the oospore hardens into nut-like structure due to deposition of calcium carbonate. Before germination, the oospore nucleus divides reductionally forming four haploid nuclei (Fig. 89B). One of these nuclei is cut off by a cell wall resulting two unequal cells.
The upper or the distal cell is small and uninucleate, the lenticular cell, whereas, the lower or basal cell is large containing three nuclei and is packed with reserve food (Fig. 89C).
The three nuclei of the basal cell soon disintegrate. The small upper lenticular cell so formed later divides vertically into a rhizoidal initial and a protonemal initial which grow in opposite directions forming a colourless rhizoid and a protonema respectively (Fig. 89D to F). The protonema soon becomes green and ultimately develops into the aerial portion of a mature Chara plant (Fig. 89G & H and Fig. 90).
Position of Chara:
Chara exhibits marked advancement:
(i) The vegetative body with highly developed aerial portion being differentiated into nodes and internodes bearing whorls of branches arising from the nodes and a less conspicuous prostrate portion resembling like root system,
(ii) great elaboration of sexual reproductive organs,
(iii) biflagellate elongated antherozoids,
(iv) oogamous sexual reproduction, and
(v) elaborate post- fertilization stages.
All these features suggest to raise the status of Chara much higher than Algae and to place closer to the bryophytes. But there are certain weaknesses which retain Chara where it is at present.
The feautures of Chara are:
(i) haploid plant body
(ii) the apparently complex nucule is actually a simple unicellular structure typical of any other alga
(iii) the diploid stage being restricted in the zygote as the first division of the zygote nucleus is reductional.
But the differences between Chara and the members of the green algae are again strong enough to put Chara in a separate class under the division Chlorophyta. Smith and others grouped Chara under a special class the Charophyceae. To Church, Chara is a remnant of the many probable evolutionary tendencies that failed to attain land habit. The fossil evidence by no means gives any clue to determine its position. Perhaps Chara represents just a mere specialized side line of the Chlorophyta.
Some Indian species of Genus Chara:
Chara benthamii Braun.; C. brae hypus Braun.; C. braunii Gmelin.; C. corallina Willd.; C.fragilis Desv.; C. gymnopitys Braun.; C. zeylanica Willd.
Special features of Genus Chara:
1. Plant body exhibits very elaborate complexity in structure and appearance.
2. The main axis and its branches are differentiated into a series of nodes and internodes. The nodes bear branches, leaves and sex organs.
3. The plant body is anchored with the substratum by rhizoids.
4. Vegetative propagation by means of specialized spore-like and protonema- like structures.
5. Sexual reproduction oogamous. The sex organs are large and highly specialized and complicated in structure.
6. The female organ known as nucule is surrounded by a protective sheath of five long spirally wound filamentous structures ending in a five-celled Grown or corona —a special mechanism by which the female organ receives protection.
7. The male organ or the globule is composed of eight plate-like cells which bear rod-like handle or manubrium, from whose inner end arise capitula ending in antheridial filaments. Each cell of the antheridial filament gives rise to a single biflagellate antherozoid.
8. Very elaborate post-fertilization stages.
Genus Nitella:
The genus Nitella has resemblance with Chara in several aspects. Like Chara, Nitella also constitutes an extensive subaquatic vegetation in small ditches and similar other areas. But unlike Chara, Nitella grows in much deeper water. The vegetative body is somewhat shorter than Chara with bushy habit (Fig. 91A). It has the appearance of miniature horse-tail (Equisetum).
The mode of sexual reproduction is very similar to Chara. There are both dioecious or monoecious species, in the latter case the globule and nucule are juxtaposed, the globule being directed upwards and the nucules downwards (Fig. 91B). Both organs appear on lateral branches of limited growth.
The nucule is relatively less elongated than what is found in Chara and bears ten-celled crown which is again much shorter than that of Chara (Fig. 91C). Besides all these major differences, Nitella and Chara are very similar to each other.
Some Indian species of Genus Nitella:
Nitella acuminata Braun.; N. flagellifera Groves & Allen.; N. furcata Agardh.; N. hyalina (DC) Agardh.; N. mucronata (Br.) Miquel; N. oligospira Br.; N. pseudoflabellata Braun.