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 zoéspores and gametes are produced in this form. Any 
cell may form zoéspores, which escape from the mother cell 
enclosed in a membrane and then arrange themselves into a new 
colony. Instead of zodéspores the cells may form gametes, which 
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Fic. 265. — Water-net, Hydrodictyon reticulatum. a, portion of a net 
(X about 2); 6, a cell which has formed zoéspores; c, the zodspores 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 zodspores 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 zoéspores or from zygospores. When 
a cell reaches a certain size and other conditions are right, its 
protoplast divides into thousands of zodéspores. These zodspores 
do not escape but, after swimming about for a time in the mother 
