GROUP I. THALLOPHYTA : ALG.E. 



259 



sentially apical, though in the discoid forms the apical cells consti- 

 tute a marginal series ; most of the cells eventually develop the peculiar 

 sheathing hairs which have suggested the name of the family. Fresh- 

 water : Coleochaete, the sole genus. 



Any cell may set free its protoplasmic contents as a zoospore with two 

 cilia. 



The sexual organs, oogonia and antheridia, are differentiated, especially 

 in the more distinctly filamentous forms. In the filamentous forms (e.g. 

 C. pulvinata, Fig. 148) the oogonia and antheridia are borne at the ends of 

 the branches ; the terminal cell of a branch enlarges to form an oogonium, 

 becoming spherical, and growing out into a long filament, the trichogyne ; 

 the antheridia are developed as small flask-shaped cells from the terminal 



Fre. 113. Coleoc'KBte pidvinita (x.350: after Pringsheim). A Part of a sexual plant 

 bearing oogonia og (with trichogynes tr) and antheridia OH ; h hair?. B portion of 

 a plant in which a mnlticellnlar structure has been developed in each fertilized oogoninm. 

 C the isolated structure the investment of which is ruptured prior to the setting free of 

 zoospores. 



cells of a filament. In the discoid forms (e.g. C. scutate), the oogonia and 

 antheridia are not terminal ; the oogonium is simply an enlarged 

 spherical cell and has no trichogyne ; the antheridium is simply a small 

 cell formed, in a group of four, by the division of one of the vegetative 



('(Us. 



A single oosphere is formed in each oogonium, and a single spermatozoid 

 in each antheridium. The spermatozoids, on being set free, find their 

 way to the oogonia, and, entering by an opening in the wall (in the 

 trichogyne when it is present), reach the oospheres and fertilise them. 



The effect of fertilisation is not only to cause the oosphere to become an 



