104 SCIENCE PROGRESS 



makes these two groups diverge collaterally from a common 

 ancestry. 



It finally remains to consider the case of the Heterokontae, 

 a group in which a distinct Flagellate origin is now well estab- 

 lished, and which is fairly sharply marked off from the other 

 green Algae (Bohlin 8 and g, Blackman 5), although Wille (70a, 

 p. 2) in his recent revision of the Chlorophycese does not accept 

 the group. The most distinctive characters of the Heterokontae 

 are furnished by the zoospores (cf. also Pascher 55), which 

 have a certain power of amoeboid change of shape, possess two 

 cilia of unequal length (or, in some cases, but a single long 

 one^), and have yellowish green chloroplasts without pyrenoids 

 (fig. 4, m), the yellow colour being due to the presence of an 

 excess of xanthophyll. Other characteristics are the absence of 

 starch, oil or soluble carbohydrates being the first assimilatory 

 products, and the presence of a considerable quantity of pectic 

 substances in the cell-wall. The best-known algal representative 

 of the Heterokontan series is the genus Conferva (Lagerheim 44), 

 and we may briefly consider it as a type. It consists of un- 

 branched filaments (fig. 4, e), which are composed of cylindrical 

 cells, the genus occurring both in fresh and salt water. Each 

 cell contains a considerable number of somewhat irregular 

 discoid chloroplasts, having the yellow-green colour charac- 

 teristic of the group, while the markedly stratified wall of the 

 cell consists of two halves, the edges of which fit over one 

 another. Since the half-pieces of adjacent cells are joined 

 together by the transverse wall, the filaments tend to break up 

 into a number of H-shaped pieces, when subjected to gentle 

 maceration or when zoospores (or aplanospores) are liberated 

 (fig. 4, n). The latter are formed to the number of one or two 

 in each cell, and show the characters mentioned above. On 

 coming to rest the zoospores exhibit amoeboid movements and 

 then become attached by their back end, while the front end 

 grows out to form a new filament. Sexual reproduction has 

 recently been described, and appears in some cases to show 

 indications of anisogamy (Scherffel 58). 



It is in the group of the Chloromonadineae (one of the algal 

 Flagellate groups) that we have Flagellate forms, which show 



' It is not improbable that a second short cilium may still be discovered in 

 these cases, as the biciliate zoospores of Conferva were originally described as 

 ijniciliate. 



