48 



PLANT MORPHOLOGY 



has been formed by the vegetative cell, while the first antheridium has 

 produced two sperms. In the male filament on the left two sperms have 

 escaped from the upper antheridium, but two more have been formed in 

 the lower one. 



Bulbochaete is a genus closely resembling Oedogonium, differing chiefly 

 in having branches, most of the cells of which bear long one-celled hairs 



that are swollen at the base. 



Summary. The Oedogoniales are a small order 

 differing from the Ulotrichales mainly in having 

 a peculiar method of cell division and motile re- 

 productive cells with a crown of cilia. The vege- 

 tative body is multicellular and filamentous, the 

 cells having one nucleus and a single chloroplast. 

 Asexual reproduction occurs by zoospores, some- 

 times by akinetes. All the members are heter- 

 ogamous. 



5. Conjugales 



The Conjugales constitute a distinct and highly 

 specialized order of green algae that occupy an 

 isolated position. In fact, they are sometimes 

 removed from the Chlorophyceae and made an 

 independent class. All of them occur in fresh 

 water. They include 38 genera and over 2,400 

 species. Some representative genera are Closterium, 

 Cosmarium, Mougeotia, Spirogyra, and Zygnema. 



Desmids. These algae are widely distributed 

 in bogs, ponds, and small lakes, usually becoming 

 abundant late in the season. They number about 

 2,250 species. Closterium is a genus of nearly 200 

 species, while Cosmarium has over 800. The des- 

 mids are unicellular and the cells display a great 

 variety of form. Like the diatoms, they have won 

 the favor of microscopists by their great beauty. 

 Desmids are typically solitary, but some develop 

 into filamentous colonies. Many desmids have the power of movement, 

 which appears to be caused by exudation of mucilage through pores in the 

 cell wall. 



In most desmids the cell is organized into two symmetrical halves that 

 are generally separated by a median constriction called the isthmus (Figs. 

 36 and 37). In each half there is usually one large chloroplast (some- 

 times two) with one or more pyrenoids. The chloroplast is often elabo- 

 rately lobed. The nucleus lies in the isthmus. In Closterium, at each 



Fig. 35. A species of 

 Oedogonium having 

 dwarf male filaments, 

 three of which have 

 developed on the cell 

 below the oogonium, 

 X300. 



