Division I. ISOKONT^ 



This group of the Chlorophyceae is on the whole a very natural one and 

 is based largely upon the constancy of the zoogonidium-characters. The 

 name was first given by Blackmail & Tansley ('02) to the large residue of the 

 Green Algae after the Stephanokontse, Akontse (Conjugatse), and Hetero- 

 kontse had been removed. Since the group is undoubtedly homogeneous, 

 it is unlikely that its general concept will be much changed in the future. 



The plant-body is unicellular, coenocytic, or multicellular ; and sometimes 

 (as in the Siphonocladiales and Hydrodictyaceo?) it is an aggregate of cceno- 

 cytes. In many forms the cells are associated to form colonies of variable 

 limitations, sometimes loose and indefinite (Tetraspora, Coccomyxa, etc.), at 

 other times compact and regular (Ccelastrum, Scenedesmus, etc.). In the 

 higher types the plant is a simple or branched filament, or even a plate- 

 like expansion (Ul vales). The branches may be free (Cha?tophoracese) or 

 concrescent (some species of Coleochwte and Cephaleuros), or interwoven to 

 form a more or less compact thallus (some forms of the Siphonales and 

 Siphonocladiales). The cell-wall is often to a considerable extent mucila- 

 ginous. Each cell contains one or more chloroplasts, and chlorophyll is the 

 principal pigment, although xanthophyll is present in variable amount. The 

 principal food-reserve is starch ,(or rarely, a fatty oil) and one or more 

 pyrenoids are generally present in each chloroplast. 



Reproduction occurs in most forms by zoogonidia of an ovoid form, to 

 the narrower and usually colourless end of which two (or more rarely four) 

 cilia of equal length are attached. Many of these zoogonidia possess a 

 laterally placed pigment spot (or stigma), and a single bell-shaped or 

 basin-shaped chloroplast usually occupies the rounder or posterior end of 

 the cell. After a variable period of activity the zoogonidium comes to rest 

 and soon germinates to form a new individual. Sometimes the zoogonidia 

 are facultative gametes, but in many forms obligate gametes also occur. 

 Such gametes are frequently similar to the zoogonidia and are known as 

 planogametes. If they are all alike they are termed isogametes, but often 

 they are differentiated into megagametes (female) and microgametes (male), 



