THE ALGAE 



83 



Oedogonium 



Included in this genus are a large number of fresh-water species which 

 frequently occur in ponds and tanks but are uncommon in running water. 

 The Alga is composed of a filament of rather elongated cells, and is usually 

 attached at one end by a rhizoid cell which is often expanded as a terminal 

 disc or holdfast. This cell contains less chlorophyll than the other cells 

 of the filament and is probably unable to assimilate food material. 



The structure of the vegetative cell is simple ; it consists of a cellulose 

 wall, bounding the inner surface of which is a layer of protoplasm. The 

 centre of the cell is occupied by a large vacuole with cell sap. The chloroplast 

 is a large cylindrical network, with several included pyrenoids formed from 

 protein granules, around which starch grains are deposited. Embedded in 

 the peripheral protoplasm of each cell is a single large nucleus. 



Growth consists in the division of the cells of the filament. The apical 

 cell does not divide, and in some species is elongated as a fine slender terminal 

 hair. Only certain cells of the filament divide, and these are distinguished 

 by the formation of an internal ring on the wall, near the upper end (Fig. 64). 



I I 



11 



Fig. 64. — Oedogonium sp. Vegetative cell on left. Stages in cell 

 division on right with nuclei only shown. {After Hiru.) 



The cell divides transversely and then the cell wall splits at the level of the 

 ring, which stretches out longitudinally to form a new portion of cell wall. 

 Another ring then forms at the upper edge of this new length of wall. Only 

 the cell which contains the ring divides again, and it possesses overlapping 



