THE FLOWERLESS PLANTS. 



41 



used. Figure 18 shows one species of the pond scums in 

 suitable condition for this study. The plant is a thallus. 

 A thallus is the name applied to any plant that does not 

 have roots, stem, or leaves. All such plants are Thallo- 

 phytes. 



The plants are one celled. They multiply by cell divi- 

 sion. The divisions take place at right angles to the 

 length of the cells, and, as the cells are covered with a 

 gelatinous coat- 

 ing, they hold to- 

 gether and form 

 long threads. 

 There must, how- 

 ever, be some or- 

 ganic difference 

 in the make-up 

 of the cells of 

 different threads, 

 for, at the proper 

 season of the year, 

 the cells of one 

 thread send out 

 little tubes to the 

 cells of some other 

 thread lying near, 

 and empty their 

 cell contents into 

 the cells of the 

 second thread. 

 The contents of 



the two cells mill- 



gle and form a 



zygospore. This becomes encased in a comparatively thick 



coating, i.e., it becomes encysted, sinks into the mud at 



the bottom of the water, and preserves the life of the 



FlG - 1. - Spirogyra. A t vegetative > cells; B, cells 

 conjugating. (X 480.) 



