CHLOROCOCCALES 



43 



one of the most remarkable that is to be found among the fresh- 

 water algae. Pediastrum is a very poor indication of what the 

 ultimate development of this type of thallus construction could be, 

 and this provides a problem at present unsolved, namely, the absence 

 of any intermediate morphological stage between Pediastrum and 

 Hydrodictyon. Further increase in the size of colony is probably 

 impossible for purely mechanical reasons. The fact that the cells are 

 coenocytic also indicates that the siphonaceous habit must have 

 arisen several times in the course of evolution. Hydrodictyon is 

 essentially a collection of a number of individual coenocytic plants 

 because it has arisen as a result of the fusion of a number of 

 swarmers. Volvox, on the other hand, must be regarded as a single 

 plant composed of a number of cells connected by strands because 

 it arises from a single zygote or asexual cell. Gamete and zoospore 

 production respectively can be obtained in Hydrodictyon by varying 

 the external conditions artificially. For example, if plants are grown 

 in weak maltose solutions in bright light or in the dark and are then 

 transferred to distilled water, zoospores will develop under the first 

 set of conditions and gametes under the second. 



CoELASTRACEAE : Sccnedesmus (scene, rope; desmus, fetter). Fig. 32. 



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

 rarely, sixteen cells. The two end cells of the chain may differ in 



G F 



Fig. 32. Scenedesmus. A, 5. acuminatus. B, S. acuminatus with mucilage 

 bristles. C, S. quadricauda. D, S. quadricauda reproducing. E-I, stages in the 

 formation of daughter coenobia in S. quadricauda. (After Fritsch.) 



