90 A TEXTBOOK OF THEORETICAL BOTANY 



the wall consists of three distinct layers, the characteristic chloroplast dis- 

 appears and the cell becomes filled with brownish oil-drops. 



The further development of the zygospore depends upon the decay of 

 the parent filament. As this consists simply of the dead cell wall, its decom- 

 position in water is fairly rapid, and in time the zygospores are set free and 

 fall to the bottom of the pond. Activity only begins when the zygospore is 

 lying in water : it will not germinate in a dried-up pond. Under suitable 

 conditions the thick wall is burst open and the contents emerge as a green 

 cell which divides transversly, the lower half forming a rhizoidal cell, while 

 the upper half divides repeatedly to form a new filament (Fig. 70). At the 



Fig. 70. — Spirogyra longata. A, Nuclear division in the zygospore. B, Abortion of three of 

 J^the four nuclei formed. S. neglecta. C, Germination of the zygospore. {After Trondle.) 



beginning of germination the nucleus of the zygospore divides into four by 

 meiosis, but only one of the four monoploid nuclei survives, to become the 

 nucleus of the first cell of the new plant. 



'Fhere is no asexual method of reproduction in Spirogyra, and this and 

 the entire absence of any truly motile gametes constitute the fundamental 

 characteristics which distinguish this genus, as well as the whole of the 

 Conjugales, from the rest of the Green Algae. It seems probable that the 

 absence of motile gametes is a feature associated with the habit of the plant 

 of living in dense masses where little or no difficulty is experienced in the 

 association of cells potentially able to conjugate, as a result of which the 

 gametes have, in the course of evolution, lost the power of motility. 



Spirogyra has, however, a method of multiplying vegetatively by the 

 fragmentation of the filaments into single cells, each of which becomes the 



