HYDRODICTYON 



309 



Pediastrum. A more complicated colony occurs in Pedi- 

 astrum (Fig. 264), another form common in ponds and other quiet 

 waters in warm weather. The cells, which are quite numerous 

 in some species, form plate-like colonies in which marginal cells 

 differ in form from those within. 



Both zoospores and gametes are produced in this form. Any 

 cell may form zoospores, which escape from the mother cell 

 enclosed in a membrane and then arrange themselves into a new 

 colony. Instead of zoospores the cells may form gametes, which 



a 



FIG. 265. Water-net, Hydrodictyon reticulatum. a, portion of a net 

 (X about 2); 6, a cell which has formed zoospores; c, the zoospores formed 

 into a small net within the mother cell; d, a cell in which gametes have 

 formed; at the left of the opening through which the gametes are escaping 

 two gametes are shown fusing. 



resemble zoospores but are smaller and more numerous. The 

 gametes, since they are alike, form zygospores, and each zygo- 

 spore upon germination produces a new colony. 



Hydrodictyon. This is the remarkable Water-net, in which 

 the cylindrical colonies, often a yard or more in length, comprise 

 thousands of cells so joined as to enclose polygonal meshes and 

 thus form a net as Figure 265 shows. These massive colonies, 

 buoyed up by bubbles of oxygen caught within them, often form 

 extensive floating mats in lakes, ponds, and sluggish streams. 



New nets may arise from zoospores or from zygospores. When 

 a cell reaches a certain size and other conditions are right, its 

 protoplast divides into thousands of zoospores. These zoospores 

 do not escape but, after swimming about for a time in the mother 



