Evolution of Plants 309 



noids — two to twenty — are formed as in Stephanosphcera and 

 Chlorochytrium, each of which becomes a starch-forming center. 



On the other hand in such genera as Chlorella, Conferva, 

 and Botrydium that make up the group Heterokontse pyrenoids 

 are not formed, and here oil instead of glycogen or starch 

 appears. Alike from the chemical and nutritive standpoints 

 this may be regarded as a transition phase in the history of 

 the green algae, since from the standpoint of molecular colloid 

 organization the oil is intermediate in chemical complexity 

 between glycogen and starch. In now forming a somewhat 

 aberrant assemblage apart from other green or from higher 

 algse, it is by no means unlikely that the Heterokontse repre- 

 sent a special advancing group from the Cyanophycese, that 

 have become a morphological side-line. 



From ancient and now extinct members of the above groups 

 several diverging lines of evolution seem to have arisen, most 

 of which have remained amid fresh- water surroundings. Liv- 

 ing genera like Schizochlamys, Nephrocytium, and Scenedesmus 

 indicate by their structure and life history transition connec- 

 tions from the Cyanophyceae to the Desmideae, and that ex- 

 tinct forms doubtless united in finely graded series. The 

 group Desmideae now consists of 31 genera and 1029 wholly 

 fresh- water species. Great diversity in advancing evolution 

 of the chromatophore is seen in these. Thus some genera 

 have a single greatly expanded peripheral mass; in others it 

 is subcentrally or centrally placed with radiating bands or 

 arms; or again two to seven peripheral or central or spirally 

 placed chromatophores occur. Equally varied are the pyre- 

 noids, which may be single and central in the mass, or two 

 to many may be distributed through its substance. 



From an ancestral desmidean stock again, the mesocarpous 

 and zygnemaceous groups have evidently started. Though 

 together they now include only six genera and 140 species of 

 wholly fresh-water habitat, these may well be taken to rep- 

 resent small remnants of two once abundant families. They 

 may show one to seven peripheral spiral chromatophores with 

 numerous pyrenoids at intervals along them, or one to several 

 central chromatophores may show a like number of pyrenoids. 



The Desmidese and two last groups seem all to form a phylum 

 that diverged into a sej^arate pathway of variation from otJier 

 algse, but which is still represented by many and world-wide 

 species. 



Another diverging line has probably passed from greenish 

 unicellular cyanophyceous ancestors tlirough protococcaceous 

 forms, allied to Endosphcera and Phyllohium to the simpler 



