44 THE ALGAE 



Ankistrodesmus and Selenastrum where the sickle-shaped cells ad- 

 here together locally through mucilage. Species of Pediastrum are 

 common components of fresh-water plankton. The cells are united 

 into flat, disc-like coenobia with up to 128 cells, the number varying 

 with the species. Experiments suggest that the shape of the cells is 

 determined by heredity and mutual pressure, but the outermost 

 cells commonly differ in shape from interior ones and possess 1-3 

 processes. Some species have tufts of gelatinous bristles. Young 

 cells are uninucleate but mature cells may possess up to eight 



nuclei. 



Asexual reproduction is by means of biflagellate zoospores which 

 are usually liberated at daybreak from the parent cell into an ex- 

 ternal vesicle in which they move about for a time, but they become 

 arranged into a new coenobium before the vesicle ruptures. In rare 

 cases long-living aplanospores are produced from individual cells. 

 Plants can also produce isogametes that fuse in pairs. The zygotes 

 increase in size and eventually divide to give a ntimber of zoospores. 

 When these come to rest they develop into thick-walled poly- 

 hedral cells. These cells eventually produce zoospores that are 

 liberated into a vesicle where they come together to form a new 

 colony. 



* Hydrodictyaceae : Hydrodictyon {hydro, water; dictyon, net). 

 Fig. 20 

 The number of species are few, the commonest, H. reticulatum, 

 having a world-wide distribution though it occurs but rarely in each 

 locality. This appears to be due to the fact that its spread depends 

 upon the germinating power of the zygotes and this only takes 

 place under favourable conditions of water quaUty. It is a hollow, 

 free-floating, cyUndrical network closed at either end and up to 

 20 cm. in length. The individual coenocytic cells are multinucleate 

 and are arranged in hexagons or pentagons to form the net. The 

 chloroplast is reticulate with nimierous pyrenoids, though in the 

 young uninucleate cells there is but a simple parietal chloroplast 

 which later becomes spiral and then reticulate. H. africanum and 

 H, patenaeforme develop into saucer-shaped nets, the former with 

 spherical cells up to i cm. diameter, which may become detached 

 and He on the substratimi where they look like marbles. The latter 

 species is composed of cells which may grow up to 4 cm. long by 

 2 mm. in diameter. Experiments have shown that 1000 lux repre- 



