250 . PART III. THE CLASSIFICATION OF PLANTS. 



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

 the more distinctly filamentous forms. In the filamentous forms (e.g. C. pul- 

 vinata. Fig. 179) the oogonia and antheridia are borne at the ends of the 

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

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

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

 a filament. In the discoid forms (e.g. C. scutata), 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 cells. 



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 



FIG. 179. Coleochcetepulvinata(x.350: after Pringsheim). A Paitof an actual gameto- 

 phyte bearing oogonia og (with trichogynes tr) and antheridia an; h hairs. B portion of a 

 plant in which a multicellular sporophyte has been developed in each fertilised oogonium. 

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

 zoospores. 



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 

 oospore by clothing itself with a proper wall, but also to cause the neighbouring 

 cells to grow round the oogonium and form a compact cellular investment for 

 it. Surrounded by this investment, the oospore falls to the bottom of the 

 water, as the plant dies down, and undergoes a period of quiescence. On ger- 

 mination it grows, splitting the investment, and divides to form a small multi- 

 cellular body, the sporophyte, the existence of which shortly comes to an end 

 by the escape of the whole of the protoplasmic contents of all the cells as 



