Phylogeny of fsokontas 157 



the latter being the more active. The greatest differentiation of gametes 

 occurs in those forms in which there are large non-motile egg-cells and 

 very small ciliated antherozoids ( Volvox, Coleochiete, etc.). The germination 

 of the zygote is very variable, and may be direct or indirect. In the coeno- 

 cytic and multicellular forms multiplication may take place by fragmentation 

 of the thallus (Monostroma, Prasiola, Caulerpa, Vaucheria, etc.). Repro- 

 duction also occurs in many of the families by akinetes or by aplanospores. 



The group of the Isokontse is of particular interest owing to the fact 

 that it contains the great majority of the most primitive known forms of 

 the Green Algae. It is only quite recently that an exact knowledge of some 

 of these forms has been acquired, and there are yet very many about which 

 the available information is of the scantiest nature. The details of the 

 arrangement of some of the lower types may, therefore, require consider- 

 able readjustment before a really natural system of classification is finally 

 built up. 



Chodat ('97), from his observations on the lower forms of Green Algse, 

 both in a state of nature and in cultures, suggested that the principal groups 

 of the Chlorophyceae all had an origin in the Palmellaceae, one of the lowly 

 families of the Protococcales. He recognized three conditions exhibited by 

 the lower Green Algse (i.e. the Protococcales) : (1) the zoospore-condition, 

 the other two conditions being only transient ; (2) the sporangium-condition 

 or unicellular motionless stage, the other conditions being realized accident- 

 ally; (3) the Tetraspora-stage where non-motile cells are connected by 

 regular cell-walls at right angles. 



Blackman ('00), following the lines first indicated by Luther and Bohlin, 

 considered that whatever tendencies were exhibited by the lower Green 

 Algse, they had their origin in the motile unicellular Chlamydomonads. 

 He defined more precisely the three divergent tendencies of the lower 

 Green Algae, originally suggested by Chodat, as follows: (1) a Volvocine 

 tendency towards the aggregation of motile vegetative cells into gradually 

 larger and more specialized motile true coenobia; (2) a Tetrasporine tendency 

 towards the formation of aggregations by the juxtaposition of the products 

 of septate vegetative cell-division to form non- motile organisms of increasing 

 definiteness and solidarity ; (3) an Endosphaerine tendency towards the re- 

 duction of the vegetative division and septate cell-formation to a minimum. 

 There is every reason to believe that these divergent tendencies had their 

 origin in the unicellular genus Chlamydomonas, and the Chlamydomonads 

 must therefore be considered as the real primitive forms of green plants. 

 The genus Chlamydomonas has been extensively studied in recent years, 

 and more than 30 species are known to occur, all of which possess more 

 or less distinctive cytological characters. Unfavourable conditions produce 

 in this genus the so-called c palmella-state,' which is the beginning of the 



