4 ZYGNEMATACEAE 



gametes is by means of a tube between the gametangia, or some- 

 times by a mere opening between the adherent gametangia. The 

 zygospores are formed either in the tube or within one of the 

 gametangia. 



FAMILY 2. MESOTAENIACEAE 



Vegetative cells are solitary, cylindric or spindle-shaped, or 

 sometimes loosely united into filaments within an amorphous 

 pectic gel or pectic sheath, and without wall pores. Chromato- 

 phores are stellate, platelike, or ribbonlike and spirally arranged. 

 Conjugation of gametes occurs through tubes of cellulose, or 

 within a pectic gel. 



FAMILY 3. DESMIDIACEAE 



Vegetative cells are solitary, or sometimes are united into 

 simple filaments. Vegetative cells have highly diversified and 

 sometimes bizarre forms, but all have transversely segmented 

 walls and wall pores. In most genera the cells have a median con- 

 striction or isthmus between the two nearly symmetrical halves. 

 There may be one or several chromatophores in each semicell 

 with a nucleus in the isthmus. During cell division, after the 

 nucleus divides, the isthmus elongates and the two semicells are 

 separated by a median wall. On both sides of this wall new semi- 

 cells develop from the isthmus. Conjugation is by gametes that 

 emerge from their respective cell walls and unite within an amor- 

 phous pectic sheath or in a conjugating tube. (For further details 

 see G. W. Prescott, "Desmids," Botanical Review, 14 [1948], 

 pp. 644-76.) 



The Family ZYGNEMATACEAE Smith 1933 



The species belonging to the Zygnemataceae are probably more 

 numerous, and are more generally distributed over the earth than 

 those of any other family of filamentous green algae. The tangled 

 green masses of algae floating on ponds, ditches, and slow streams 

 anywhere are sure to contain representatives of this group. 



All the species have cylindric cells during the period of vege- 

 tative growth. During the reproductive period, however, the cell 

 walls of some species grow and change their size and form in a 

 most astonishing manner. In some species the gametangia alone 

 are affected. The receptive gametangia in particular may become 

 distended on all sides, on the conjugating side only, or only on 



