Evolution of Plants 313 



But the above six families, T\dth some smaller related ones 

 which cannot be individually considered here, show graded 

 connection in the number of chromatophores, from one large 

 expanse that almost encircles the cell, to another that is reticu- 

 late and may separate into parts as in Chcetomorphay or numer- 

 ous small subdivided bodies that closely line the peripheral 

 protoplasm as in Conferva. Simultaneously the single pyrenoid 

 of the simpler genera multiplies by degrees into many that 

 dot over the chromatophore substance, except in such a group 

 as Vaucheriacese, where oil instead of starch formation takes 

 place, and the pyrenoids are absent. 



We now reach two families that seem to the writer of excep- 

 tional importance, as being probably related on the one hand 

 to some of the higher Cyanophycese, and on the other to the 

 simplest conceivable Bryophyta and Pteridophyta. We refer 

 to the Chsetophoracese and Coleochsetacese, that mil be treated 

 separately. Amongst the Blue-green Algae the family Cliam- 

 sesiphonacese deserves special attention, in that most of the 

 included genera produce conidiospores inside conidangia, that 

 are some multiple of four. Each conidiospore is passive and 

 non-ciliate. But as Phillips has shown, and as the writer 

 has often observed, there is a strong tendency toward produc- 

 tion of semi-ciliate waving processes in the end-cells of many 

 Cyanophycese, as well as fine intercellular threads, either of 

 which may well represent rudimentary ciliary formation. If 

 then the conidial cells before being set free retained or devel- 

 oped such processes, swarm-cells would result. 



Now in cytological aspect and structure of the entire plant, 

 there is striking agreement between some chamsesiphoneous 

 genera and those of the Chsetophoracese, the presence of a 

 distinct nucleus in the latter being the main difference. In 

 Microthamnion and Phceothamnion also the color is yellow- 

 green, blue-green, or yellow-brown. In this family then, we 

 might look for a common point of divergence from the Cyan- 

 ophyceae to the higher green and the brown algae. The diver- 

 sified structure, aspect, and environal connection of the thallus 

 in the family suggest that the living genera are scattered rem- 

 nants of a greatly larger assemblage, which in remote past 

 periods were much richer in connecting genera. The single 

 or more or less subdivided chromato])hore may readily in 

 higher derived types have become distinct small chlorophyll 

 bodies. 



But a feature of special interest is the structure of the swarm- 

 spores, and their often exact similarity to sexual or gamete 

 cells. 



