The following points highlight the four methods of plant reproduction. The methods are: 1. Vegetative Reproduction 2. Asexual Reproduction 3. Sexual 4. Induced Sex Modification.
Contents
Method # 1. Vegetative Reproduction:
A. Natural Methods of Propagation:
(a) In lower plants:
(i) By budding:
Experiment:
Some yeast are grown in sugar solution and observed under the microscope from time to time.
Observation:
It is observed that one or more tiny outgrowths appear on one or more sides of the vegetative cells immersed in sugar solution. In some cases the outgrowths may be detached from the mother cell (which will grow into new individuals).
Inference:
This method of outgrowth formation is known as budding. Often budding continues one after the other so that finally a chain of cells is formed. All the individual cells of the chain separate from one another and form new yeast plants.
(ii) By gemmae:
Experiment:
Some gemmae cups are collected from Marchantia and placed in its natural habitat.
Observation:
A new thallus of Marchantia is grown from a gemma cup.
Inference:
This is a special method of vegetative reproduction. The gemmae develop in the gemmae cup and each gemma is a small, more or less circular flattened structure with a conspicuous depression on each side. The growing point lies in the depression.
(iii) By leaf tip:
Experiment:
The leaf tip of adiantum is made to touch the soil in which it grows and after a few days observation is made.
Observation and inference:
When the leaf bends down and touches the ground the tip strikes root and forms a bud. The bud grows into a new independent fern plant.
(b) In higher plants:
(i) By underground stems:
Experiment:
Some tubers of potato or bulbs of onion or rhizomes of ginger are planted in pots. Observation is made after 10 to 15 days.
Observation:
Many flowering plants reproduce themselves by means of rhizome, tuber, bulb or the corm. New buds are produced on these modified stems which gradually grow up into new plants.
(ii) By sub-aerial stems:
Experiment:
Some Pistia, Hydrocotyle, Colocasia or Chrysanthemum plants are observed.
Observation and inference:
Here vegetative propagation takes place by means of sub-aerial stems, e.g., by runner in Hydrocotyle, by offset in Pistia, by stolon in Colocasia and by sucker in Chrysanthemum.
(iii) By Adventitous buds:
Experiment:
Some Bryophyllutn leaves are kept in moist soil.
Observation and inference:
A series of adventitious foliar buds arc produced on the leaf margin (notch), each at the end of vein. These buds grow up into new plants.
(iv) By Bulbils:
Experiment:
In Globba bulbifera or Allium sativum some of the lower flowers of the inflorescence become modified into small multicellular bodies, known as bulbils (Figure 44). Some bulbils are kept on moist ground and observed after a few days.
Observation and inference:
Bulbils grow up into new plants (sometimes they are seen to grow to some extent Fig. 44 on the plant itself).
B. Artificial Methods of Propagation:
(i) By cuttings:
(a) By stem cuttings:
When some stem cuttings of rose, china-rose, Moringa, Coleus, etc., are put (the physiological polarity should be maintained) into moist soil they strike roots at the base and develop adventitious buds which grow up.
N.B. The rooting of the stems may be hastened by treatment of the cuttings with growth hormones like IBA, NAA, NOA, etc. in lanolin paste or in solution.
(b) By root cuttings:
When some root cuttings of lemon. Citrus, tamarind, etc. are put into moist soil they sprout forming roots and shoots.
(ii) By layering:
In this case a lower branch of lemon, Ixora, rose, jasmine, etc., is bent down, a ring of bark to the length of 1 to 2 inches are removed and this portion is pushed into the soft ground keeping the upper part free. The bent portion is covered with soil and Removed bark a stone or a brick is placed on it (Figure 45).
When roots have developed (usually within two to four months), the branch is cut out from the mother plant and grown separately.
(iii) By gootee:
This method is usually employed for propagating lemon, orange, guava, and litchi. Magnolia, etc. During early rains a healthy, somewhat woody branch is selected and a ring of bark (1 to 2 inches in length) is sliced off from it.
A sufficiently thick plaster of grafting clay (clay two parts, cow dung one part and some finely-cut hay mixed with water) is applied all-round the ringed portion which is then wrapped up with straw (or polythene bag) and tied in a secured manner (Figure 46).
It should be wetted with water every morning and afternoon or may be kept moistened with the help of a wick from a water reservoir. Usually within one to three months the gootee is ready as is indicated by its striking roots. It is then cut out below the bandage and grown separately.
(iv) By grafting:
(а) Inarching or approach grafting:
By this method a branch (scion) of a plant is made to unite with a seedling (stock) by firmly tieing them together by means of a chord (Figure 47).
Before doing this, a small portion of the bark is sliced off from each to ensure a closed contact and quicker union between the two. When proper fusion has taken place (usually within two to three months), the stock is cut-out above the joining and the scion below, thus leaving the scion standing on the stock. Some of the fruit trees like mango, litchi, guava, plum, etc., readily respond to this method.
(b) Bud grafting:
For this method a T-shaped (Figure 48) incision is made in the bark of the stock, and a bud, cut out clean from a selected plant, is inserted into the T-shaped slit and properly bandaged. By this method it has been found possible to grow several varieties of roses on one rose stock good varieties of orange, lemon, etc., on inferior stocks, several varieties of China rose on one, cacti on one and so on.
(c) Whip or tongue grafting:
The stock, usually half to three-fourth inch thick, is cut down a few inches above the ground, sloping cuts are then made in it 2 or 3 inches long, as shown in Figure 49.
Scion of the same thickness is also cut in such a ways to fit exactly into the stock. It is then inserted into the stock and tied firmly. The wound is of course covered with grafting wax (a mixture of tallow (animal fat) one part plus bees wax one part plus resin four parts, melted together and made into a soft dough under water). All buds are removed from the stock but not from the scion.
(d) Wedge grafting:
The stock is cut 8 to 10 inches above the ground and the wood of the Stem incised with clean cut in the form of a ‘V’. The scion, cut obliquely downward so as to closely fit into the stock, is inserted into the stock and tied firmly (Figure 50). Grafting wax is used for covering the wound. All buds are removed from the stock but not from the scion.
(e) Crown grafting:
An old tree may be rejuvenated by this method. The stem is cut across 8 to 10 inches above the ground. The bark of the stock is cut through from the surface downward to a length of 5 to 6 inches.
The bark is partially opened on either side. Prior to this a small branch cut out from a tree is incised at the base with a sloping cut and this is now inserted into the slit in the bark and tied firmly. The wound is covered with grafting wax (Figure 51).
Method # 2. Asexual Reproduction:
A. By Fission:
In many unicellular algae, fungi and bacteria, the mother cell splits into two new cells. The new cells, thus formed, contain all materials of the mother cell and soon they grow to the size of the latter, becoming a new independent plant. This process of fission may be observed in case of yeast cells under the microscope.
B. By Spore Formation:
(i) Motile spores:
Zoospores of algae like Ulothrix, Vaucheria, etc., may be observed under the microscope. The zoospores after escape from the mother cell germinate giving rise to new filaments.
(ii) Non-motile spores:
Non-motile spores of some algae, e.g., Nostoc, Penicillium, Phytophthora,
etc., are well adapted for dispersal by wind and at the same time to meet the ever-changing conditions of the environment. In favourable conditions they germinate and give rise to new plants.
(iii) True spores:
True spores are always borne by sporophytes of bryophyta and pteridophyta. The germination of these spores may be observed in the laboratory under suitable culture conditions.
Method # 3. Sexual Reproduction:
A. By Conjugation:
In lower algae and fungi the pairing gametes are not differentiated into male and female (isogamous). The union of such similar gametes (conjugation) forms the zygote called zygospore. Conjugation of Spirogyra or Mucor may be observed under the microscope. Both scalariform and lateral conjugation may be observed in Spirogyra and only scalar form in case of Mucor.
B. By Fertilisation:
In higher plants the uniting gametes are differentiated into male and female and the union of the two (fertilization) forms the zygote (oospore). When some pollen grains are dusted on the stigma of the flower, the pollen grains germinate forming a tube like outgrowth called pollen tube- The pollen tube contains sperms which unite with the egg of the nucleus.
The pollen tube grows through the style and ultimately reaches the ovule. The growth of the pollen tube may be studied by cutting and staining the serial longitudinal sections of the style at different time intervals.
Method # 4. Induced Sex Modification:
A. By Growth Hormones:
IAA and other synthetic growth substances like 2, 4-P or NAA seem to be effective the induction and modification of sex organs.
(i) Conversion of male flowers into female flowers by NAA:
In dioeciously species of Cannabis saliva genetically male plant could be induced to produce female flowers if during the period of differentiation of flower buds the third and fourth leaves of male plant are treated with 50 ppm NAA and also by 900 ppm ethrel.
This indicates that the sexuality in this plant is determined by the concentration of native auxin and ethylene during the period of primordium differentiation and that femaleness is associated with a relatively high auxin level.
(ii) Increased production of female flowers by NAA:
In some monoecious cucumber plant painting of the lower surface of’ the leaf with 50 ppm NAA causes an increase in the proportion of female flowers, sometimes altogether suppressing the production of male flowers. This also indicates that the female flowers tend to differentiate under higher concentration of auxins than do male flowers.
(iii) Promotion of maleness in sex expression by GA:
Gibberellin increases the staminate flowers in some cases, e.g., cucumber in contrast to promotion of femaleness by auxin.
B. By Growth Retardants:
Growth retardants also modify sex expression. The growth retardants are not the causative agents of sex expression in flowers. But the influence they exert may be due to differently regulated vegetative growth, and antagonistic reactions with growth promoters.
Inhibition of Staminate flowers:
The synthetic growth retardants such as CCC (2-Chlorocthyl trim ethyl ammonium chloride) and its derivatives MAB (Methyl ammonium bromide) and BCB (2-Biomoethyl trim ethyl ammonium bromide) and Ethrel (2-Chloroethyl phosphoric acid) all these inhibit staminate flower production and greatly increase pistillate flower formation in cucumber and spinach.
The greatest increase in pistillate flower formation is caused by MAB under high light intensities.
C. By Other Methods:
(i) By carbon monoxide (CO) gas:
In some Euphorbiaceae CO gas .in low concentration sometimes reduces the number of male flowers in genetically monoecious types. The effect is presumably due to the effect of CO on auxin.
(ii) By Salinity:
Increase in salinity of the medium, particularly NaCL, may change the expression of sex in plants. In dioeciously species, salinity may sometimes significantly cause an increase in the production of staminate flowers.
(iii) By Photoperiod treatment:
The length of photoperiod also affects the differentiation of sexes. If seeds of some Euphorbiaceae are grown in short days instead of long days (in long days the plants are predominantly staminate) the percentage of pistillate plants significantly increases.
Similar results are also obtained with unisexual plants of Ambrosia of composite. Cannabis sativa growing under 16 hours photoperiod produces about half staminate and half pistillate plants. But if the day length is shortened to 8 hours the plants are about half bisexual, half females and none males. Long days seem to favour production of male sex organs.