THE POND SCUMS AND DESMIDS 



193 



OEDER 3. THE POND SCUMS AND DESMIDS, OR CONJUGALES 



223. The pond scums and desmids. The pond scums and 

 desmids (order Conjugates) are remarkable for the beauty and 

 symmetry of their cell structure, and especially for their large 

 and complicated chromatophores. There are no motile stages in 

 their life histories, and the sexual processes consist in the union 

 or conjugation of similar nonmotile gametes (isogamy). These 

 characters distinguish the group sharply from all other algse, but 



FIG. 186. Desmids 



A, Closterium, a vegetative cell at the left and a zygospore at the right between 

 the halves of two empty cells whose contents have fused ; B, Cosmarium; the 

 desmid at the right has just divided and is forming two new parts between the 

 old halves of the parent cell. C, Micrasterias, a very elaborate form in its out- 

 line and markings ; D, Hyalotheca, a common filamentous desmid ; the appear- 

 ance of the cells in face view is shown at the right 



make the relationships of the forms very uncertain. These plants 

 live only in fresh water and seek the sunshine, being abundant 

 in clear, shallow pools. The desmids live chiefly along the mar- 

 gin and on the bottom, while the pond scums frequently form 

 growths upon the surface of the water, which appear frothy 

 because of the bubbles of gas (largely oxygen) held among the 

 filaments. The filaments are very slippery to the touch on 

 account of a slimy excretion from the cells. 



224. The desmids. There are about one thousand species in 

 this large group whose forms are generally one-celled, although 

 some desmids are filamentous (Fig. 186, D). Each cell has two 



