Volvocede 



175 



relatively very small, the canals can scarcely be detected. In most of the 

 genera the cells are all equivalent and capable of reproducing the plant by 

 division, but in the higher types there is a differentiation, some cells being 

 purely vegetative, having lost the power of division, whereas others are solely 

 reproductive. 



Gonium is the simplest of the Volvocea?, either 4 (G. sociale and G. lacustre ; 

 fig. 103 B F) or 16 (G. pectorale; fig. 103 A} cells forming a flat, plate-like 

 coenobium. The amount of mucus around the colony is relatively small, and 

 the ovoid Chlamydomonadine cells are all turned one way, so that the cilia 

 are all on one surface of the plate. Each cell is also enveloped in a special 



Fig. 103. A, Gonium pectorale Miill. ; B F, Gonium lacustre G. S. West. E and F 

 show formation of daughter-colonies in mother-cells. All x 475. 



rather denser mucilaginous coat, small projections from which unite the 

 adjacent cells of the colony. In fact, Gonium is merely the aggregation side 

 by side of either 4 or 16 Chlamydomonadine cells to form a permanent colony. 



Harper ('12) has given reasons, based on careful study of the development of Gonium 

 colonies, for believing that forms and space relationships of the cells are due rather to the 

 mechanical interaction of the cells, regarded as colloid droplets, during the growth of the 

 colony, than to any predetermined hereditary factor. 



In Pandorina the aggregation of cells is entirely different, from 8 to 32 

 (usually 16) forming a compact, approximately spherical colony (fig. 104^), 

 which exhibits a distinct polarity. The cells are broadly ovoid in .shape, often 

 a little angular by pressure of contact, and with the narrower ends towards the 

 centre of the colony. A tough and rather close mucous investment surrounds 

 the coenobium, and sometimes a wider, less dense mass of jelly is also evident. 



In Eudorina the ccenobium consists, as a rule, of 32 c-lls, distantly 



