22 
MORPHOLOGY 
one of the most remarkable of plant colonies. This hollow net is freely 
floating, and sometimes attains a length of 25 to 30 cm. It is formed 
by an end-to-end union of long 
cylindrical cells in such a way as 
to form polygonal meshes, three 
or four cells abutting at each junc- 
tion (fig. 45). The cytoplasm 
forms a thick layer inclosing a 
large central vacuole, and the 
FIGS. 38, 39 .PedMstrum: cells with chloroplast is an irregular plate 
daughter colonies in different stages of ., , ,. ,, , , 
formationAfter A. BRAUN. that finall y breaks U P mto man 7 
small chloroplasts. 
When the cell reaches a certain size, the protoplast divides into very 
numerous (7000 to 20,000) biciliate zoospores, each with a nucleus and 
a small chloroplast (fig. 46). These zoo- 
spores do not escape, but swim about 
freely for a time within the large mother 
cell, come to rest, and gradually develop 
a small net within the mother cell (fig. 47). 
The wall of the mother cell finally softens 
and the young net is set free and grows to 
adult size without any cell division. 
The sexual reproduction is isogamous, certain cells producing a re- 
markable number of gametes (30,000 to 100,000), which escape from 
the mother cell at once and fuse in pairs, forming zygospores (figs. 48- 
50). Experiments upon Hydro- 
dictyon have succeeded in secur- 
ing at will the production of 
zoospores (forming young nets) 
40 41 42 43 44 
FIGS. 40-44. Pediastrum : 
40, zoospore; 41, gamete; 42, 
43, gametes fusing; 44, zygo- 
spore. After WEST. 
FIG. 45. Hydrodictyon : arrange- 
ment of cells in forming the net. 
After WEST. 
FIG. 46. Hydrodictyon: zoospores within 
a mother cell beginning to form a net. After 
KLEBS. 
