In this article we will discuss about the vegetative body and reproduction of zygnema.
Vegetative Body of Zygnema (Fig. 49A):
The thallus, the Spirogyra, is an unbranched filament without any distinction into base and apex. A filament consists of a single row of similar cylindrical cells, usually having lengths slightly greater than breadths; occasionally, the cell may be 2-5 times as long as broad. Holdfasts or haptera are rarely formed. The wall structure is much the same as that in Spirogyra, but the outermost pectose layer rarely becomes thick.
The transverse walls are never replicated. The protoplast is uninucleate, and the nucleus is embedded in a broad strand of cytoplasm connecting two chloroplasts on either side- this band occupies the longitudinal axis of the cell.
Each chloroplast is stellate or star-shaped with a prominent pyrenoid at its centre, and it may have numerous delicate strands extending towards the periphery of the cytoplasm. The filament increases in length by cell divisions and subsequent growth of the individual cells.
Prior to the cell division, the nucleus divides mitotically into two, so that after cytokinesis, each daughter cell receives one daughter nucleus and a chloroplast of the parent cell, the former lying lateral to the latter. Subsequently, the chloroplast with its pyrenoid divides into two and the nucleus migrates and occupies a position midway between the two daughter chloroplasts.
Reproduction in Zygnema:
Zygnema reproduces by vegetative and sexual methods. The vegetative reproduction usually takes place by the accidental breaking of the filament and never due to the softening of the cross wall of the adjacent cells.
Most species of Zygnema reproduce sexually, in the spring, by the process of conjugation (Fig. 49B). Conjugation is usually scalariform, but lateral conjugation is not of rare occurrence. The process of scalariform conjugation is much the same as that in Spirogyra.
The gametes of one filament (male) become active, amoeboid and migrate through the conjugation tubes to the gametes of the other filament (female), unite and form zygospores. But in some species both the gametes become amoeboid, migrate towards each other, fuse in the conjugation tube and form a zygospore there.
After gametic union, the zygote is invested by a wall, which becomes three-layered at maturity (a thin inner layer of cellulose, a median layer of cellulose which is often chitinized, and a thin outer layer of cellulose or pectose). The colouration and ornamentation are restricted to the middle layer, as in Spirogyra.
The zygotes at maturity, are finally liberated by the disintegration of the gametangial wall or of the wall of the conjugation tube, and usually undergo a period of rest for several months. With the advent of the following spring the zygote germinates, and prior to this germination there is a reduction division of the zygote nucleus forming four haploid nuclei, of which three disintegrate.
During germination the two outer walls of the zygote rupture and the protoplast being still enclosed by the innermost wall-layer may partially or wholly escape from the ruptured walls. This protoplast by repeated divisions gives rise to a new filament.
When conjugation fails, a gamete may form azygospore or parthenospore. But these are not produced in abundance in the genus.
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