420 PRACTICAL BOTANY 



cells, with thin cell-walls, a single nucleus, and a single 

 flattened chlorophyll-body (chromatophore). 



2. The arrangement of the cells in radiating and 

 bifurcating series, which are however in close contact 

 laterally one with another, and thus compose a con- 

 tinuous disk. 



3. The hairs, which are borne by many of the cells : 

 they are long and narrow, and are sheathed at the 

 base. 



According to circumstances, the following reproductive 

 organs may or may not be found : 



1. Swarm-spores, each of which is formed from 

 the contents of a single cell of the thallus, by contrac- 

 tion, and escape through a hole in the cell- wall into the 

 water : the swarm-spores are spherical primordial cells ; 

 they move for a time by means of a pair of cilia. 



2. Antheridia, which are formed by division of cells 

 of the disk into four : the contents of these escape as 

 ciliated spermatozoids. In other species they may 

 be formed from terminal cells of the filaments 

 (0. pidvinata). 



3. Oogonia, flask-shaped cells, with long tubular 

 necks : each oogonium contains one ovum. 



4. The fructification, which results partly from the 

 maturation of the oogonium after fertilization (oospore), 

 partly from its investment by filaments which grow from 

 surrounding cells, forming a sheath one layer of cells in 

 thickness, and of a brown colour when ripe. In this 

 state the winter is passed. 



5. In specimens observed in spring the general out- 

 line of the thallus will be seen as before, but the cells, 

 excepting those of the fructification, have lost their 



