40 



CHLOROPHYCEAE 



into two, four, eight or sixteen autospores. Several species often 

 form a symbiotic association with lower animals when they are 

 known as Zoochlorella or Zooxanthella (cf. p. 296). The species are 

 frequently indeterminate systematically and are chiefly studied by 

 means of laboratory cultures, but in spite of these systematic 

 difficulties they are common objects for physiological experiments. 



Hydrodictyaceae : Pediastnun {pedia, plain ; astrum, star). Fig. 30. 



The species of this genus are common components of fresh- 

 water plankton. The cells form disk-like coenobia, the plane-faced 

 or lobed cells being arranged in one layer, experiments suggesting 



Fig. 30. Pediastrum. A, P. Boryanum ( x 333). B, P. simplex var. duodeniarum 

 ( >< 333)- C, P. Boryanum var. granulatum showing liberation of zoospores. 

 D, P. duplex with hypnospores. E, P. Boryanum, germination of tetrahedron. 

 F, P. Boryanum var. granulatum, formation of new plate. (A, B, after Smith; 

 C-F, after Fritsch.) 



that the shape of the cells is determined by heredity and mutual 

 pressure. At certain stages in the life cycle they bear tufts of 

 gelatinous bristles which are probably a modification for their 

 floating existence. There are 2-128 cells in each coenobium, 

 varying with the species, and whereas the young cells are uni- 



