624 SCIENCE PROGRESS 



largely freshwater or subaerial, while the two remaining groups 

 are almost entirely marine. It is obviously only among the 

 green Algae (in which the chlorophyll in the plastid is not 

 obscured by the deposition of a strange pigment, as is the 

 case in the other three groups) that we can expect to find 

 hints as to the ancestry of the higher representatives of the 

 vegetable kingdom, and it is interesting to note in this 

 connection that they include a far larger number of what may 

 be regarded as simple and primitive forms than the remaining 

 three groups of the Algae. It is therefore with the green 

 Algae alone that we shall be concerned in the present 

 article. 



The investigations of the last fifteen years have very 

 considerably modified the conceptions of the green Algae 

 which existed when they were all classed together as 

 Chlorophyceae. In particular two points have become evident — 

 viz. that we must distinguish at least two perfectly distinct 

 classes (Isokontae and Heterokontae) among them, and that they 

 are linked by intermediate forms to the group of the Flagellates, 

 at present still a rather motley assemblage of incompletely 

 known forms, which show a curious mixture of plant and 

 animal characteristics and are probably the survivors of the 

 simplest groups of organisms from which both kingdoms have 

 arisen. It is the purpose of this article to indicate briefly the 

 diverse lines of evolution that can at present be traced among 

 the green Algae and to consider their connection with the 

 Flagellates. Before turning our attention to these matters 

 it will be well, however, to indicate briefly the salient features 

 in the structure and life-history of the forms under con- 

 sideration. 



The green Algae comprise unicellular and colonial forms 

 (fig. i, a, fig. 2, b) among their simpler representatives, while 

 the higher forms are either branched or unbranched chains of 

 cells (so-called filaments, fig. 3, g), flat plates of cells or (among 

 the Siphonales) complicated developments of multinucleate 

 cells (so-called ccenocytes). In the simpler forms each cell 

 usually contains but a single chloroplast, which is of large size 

 and variable shape, although generally conforming to a common 

 ground plan within one and the same group. In many cases 

 the chloroplasts contain one or more pyrenoids (fig. 1, a, p), 

 i.e. specialised bodies, which are lodged within the substance 



