CRYPTOGAMS 



175 



Desniids. 



422. Conjugation of similar unciliated reproductive 

 cells is characteristic of a considerable group of Green 

 Algse. Fresh water preparations very often contain 

 unicellular forms belonging to this group, more 

 or less resembling the species represented in Fig. 

 288. Sometimes they cohere in chains. Usually 

 they are capable of slow locomotion. They are 

 Desmids. 



423. Vaucheria. — The green filaments 

 of Vaucheria are large enough to be dis- 

 tinguished by the naked eye. By repeated 

 branching they form upon moist soil 

 matted growths which may be several 

 inches in diameter. The plant also grows 

 submerged in water. The filaments are continuous tubes, 

 ordinarily without cross partitions (^.6. unseptate'), and 

 are lined with a protoplasmic layer in which numerous 

 nuclei and small rounded chromatophores are held; the 

 main cavity of the tubes being filled with cell sap as in 

 the case of Spirogyra cells. In fact the thallus of Vau- 

 cheria is to a certain degree such as would be produced 

 if the cells of Spirogyra were not separated by end walls, 



the chief differences in this respect being 

 the greater number of nuclei, the shape 

 of the chlorophyll bodies, and the 

 branching habit of Vaucheria. 



424. Reproduction. — Zoospores are 

 produced in the ends of side branches 

 after these portions have been cut off 

 by septa and thus converted into zoospo- 

 rangia. The whole contents of each 

 zoosporangium escapes by the rupture 

 of tlie wall at the apex (Fig. 289), and 

 constitutes a single large zoospore pro- 

 vided with numerous pairs of cilia distributed over its 

 surface. The motile period may last for several hours, 

 after which time the cilia are lost, a wall is formed around 

 the zoospore, and germination very soon takes place by 



289. Zoospore and 

 zoosporangium 

 of Vaucheria. 

 — Murray. 



