EUPHYCOPHYTA 



47 



CoELASTRACEAE : Scenedesmus {scene, rope; desmus, fetter). Fig. 21 



The planktonic colonies are composed of four, eight, or more 

 rarely, sixteen cells attached to each other at one point by mucilage 

 pads. This type of colony has probably originated from that in 

 which cells are attached at one point indiscriminately, as in An- 

 kistrodesmus. The two end cells of the chain may differ in shape from 

 the others and often have processes which are elaborations of the 



Fig. 21 Scenedesmus. A, 5". acuminatus. B, 5. acuminatus with 

 mucilage bristles, C, 5. quadricauda. D, 5". quadricauda reproduc- 

 ing. E-I, stages in the formation of daughter coenobia in S. 



quadricauda. (After Fritsch.) 



mucilaginous cell envelope: these processes are probably to be 

 correlated with the planktonic mode of life, whilst tufts of bristles 

 performing the same function and similar to those of Pediastrum 

 are also recorded. 



Protosiphonaceae : Protosiphon (proto, first; siphon, tube). Fig.22 

 This family has in the past been placed in the Siphonales (see 

 p. 96) but in respect of that order it possesses several anomalous 

 features, i.e. method of reproduction, haploid condition, type of 

 environment, plastid, pigments, etc. and it has now been trans- 

 ferred to the Chlorococcales. 



There is probably but a single species, P. botryoides, but it exists 

 in a number of different strains, one from the deserts of Egypt 

 tolerating temperatures up to 91° C and salt concentrations up to 

 I per cent. It commonly grows associated with a very similar alga. 



