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, sg.—Pediastrum: cells with chloroplast is an irregular plate 



daughter colonies in different stages of ., . ^ ,, , , . ^ 



formation.-AfterA.BRAaN. that finally breaks up mto many 



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- . 1 



dictyon have succeeded in secur- ^ ' if/ 



ing at will the production of 

 zoospores (forming young nets) 



40 4! 42 43 44 



Figs. 40-44. — Pediaslriim: 

 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. 



