Desmids 



117 



cystis, fig. 40 e), but there is a single nucleus, and it lies 

 in the midst of the transparent crossband. The larger 

 ones, such as Closterium (fig. 38) may be recognized with 

 the unaided eye, and may be seen clearly with a pocket 

 lens. Because it will grow per- 

 ennially in a culture jar in a 

 half -lighted window, Closterium 

 is a very well known labora- 

 tory type. 



Division is transverse and sep- 

 arates between the semicells. 

 Its progress in Closterium is 

 shown in figure 39, in a series of 

 successive stages that were photo- 

 graphed between 10 P. M. and 3 

 A. M. Division normally occurs 

 only at night. 



In a few genera {Gonatozygon, 

 (fig. 40a) Desmidium, etc.) the 

 cells after division remain at- 

 tached, forming filaments. 



Desmids are mainly free float- 

 ing and grow best in still w^aters. 

 They abound in northern lakes 

 and peat bogs. They prefer the 

 waters that run off archaean 

 rocks and few of them flourish 

 in waters rich in lime. A few 

 occur on mosses in the edges of 

 waterfalls, being attached to the 

 mosses by a somewhat tenacious gelatinous invest- 

 ment. One can usually obtain a fine variety of 

 desmids by squeezing wisps of such water plants 

 as Utricularia and Sphagnum, over the edge of a 

 dish, and examining the run-oft\ The largest genus of 

 the group and also one of the most widespread is 



Fig. 39. Photomicrographs 

 of a Closterium dividing. 

 The lowermost figure is 

 one of the newly formed 

 daughter cells, not yet 

 fully shaped. 



