Division IV. HETEROKONT^] 



It is to Luther ('99) that we owe the establishment of the group 

 Heterokontse to embrace a varied assortment of Green Alga3 which were 

 previously scattered throughout the different families of that section of the 

 Chlorophyccte now well-established as the Isokontae. 



Our knowledge of these Algse has increased very much during the past 

 few years and it is now possible to discuss their inter-relationships with some 

 degree of certainty. They are of a prevailing yellow-green colour owing to 

 the presence of a greater amount of xanthophyll than occurs in other groups of 

 the Chlorophyceie. The chromatophores are parietal, often discoidal, and are 

 destitute of pyrenoids. There are usually several or many in each cell. 

 Starch is absent and the stored product of photosynthetic activity is a fatty oil. 

 In other respects the cytological details are similar to those described for the 

 Isokontse. 



The group contains unicellular, colonial, filamentous and coenocytic types, 

 and it is possible to institute a comparison between these and corresponding 

 types in the Isokontge. 



Vegetative multiplication occurs in a few genera by the dissociation of 

 large colonies into smaller aggregates which soon increase in size by further 

 cell-division. 



Asexual reproduction takes place by ovoid or pear-shaped zoogonidia, 

 which are furnished with two cilia of unequal length. The latter are often 

 attached rather to one side of the anterior extremity of the zoogonidium. 

 Moreover, the zoogonidia possess, as a rule, more than one chromatophore. 

 Aplanospores are frequently formed in some genera, either singly or several 

 in each cell. 



Gamogenesis occurs in a number of genera and the isogametes are 

 biciliated. It is not improbable that exact observations and accurate methods 

 will demonstrate that the ciliation of the gametes is similar to that of the 

 zoogonidia, but so far such observations as have been made are few in number 

 and not sufficiently precise 1 . 



1 It should be remembered that the second short cilium was not clearly demonstrated until 

 1898, previous to which date the zoogonidia and gametes of these Algse were almost invariably 

 described as possessing only one cilium. The short cilium is usually carried in a backward 

 direction, pressed more or less closely against the body of the motile cell. 



w. A. 26 



