22 A TEXT-BOOK OF BOTANY. 



tion is through the formation of daughter colonies within the 

 mother colony, and these after a time develop motile cells like the 

 parent, which swim about and finally escape. A sexual method 

 of reproduction also occurs in which there is a union of cells 

 within the spheres, the resulting cells after germination forming 

 swarm spores that cling together to form a new colony. 



Hydrodictyon, or Water Net, is a form often very abundant 

 in sluggish and stagnant waters. It consists of a number of cells 

 forming a net, the meshes of which are usually hexagonal or 

 pentagonal in shape, depending on the number of cells outlining 

 them (Fig. u). The cells are all alike, cylindrical in form, 

 attaining sometimes a length of i cm., and usually contain a 

 number of nuclei. The green chromatophore occurs in a plate 

 at the periphery of the cell and usually contains numerous 

 pyrenoids. 



The asexual reproduction is by means of zoospores which 

 are formed simultaneously in large numbers, sometimes number- 

 ing many thousands in each cell. These zoospores as formed 

 inside of the mother cell show more or less definite movement 

 and arrange themselves finally to form a new net. The sexual 

 reproduction is characterized by several stages, (i) Some of the 

 zoospores are liberated through a pore in the cell- wall of the 

 mother cell and after swimming around for some time pairs of 

 them unite, forming zygospores. (2) After a resting period each 

 zygospore develops 2 to 5 zoospores, which escape into the water 

 and develop into irregular, sharp-angled cells, called polyhedra, 

 which persist through the winter. (3) When these polyhedra 

 develop, small zoospores are again formed, and these arrange 

 themselves to form a net inside of the polyhedron, which then 

 escapes and increases in size. 



Vaucheria (Fig. 12) is another common green alga which 

 may also be selected as showing the habits of this group of 

 plants. The plant has a branching thallus and lives in shallow 

 water or on moist earth, being attached to the substratum by 

 means of delicate root-like processes sometimes spoken of as 

 rhizoids (Fig. 12, w). In the thin layer of protoplasm lying near 

 the wall are numerous nuclei and small oval chromatophores. 



