Natural History 637 



plants. Water plants branch vigorously, and the branches, torn 

 apart, start life on their own. The water thyme, introduced from 

 America to two or three places in England in the forties, is now 

 spread over the whole country ; at various places and times it has 

 been a serious menace to navigation. This enormous multiplica- 

 tion has been entirely vegetative. The male and female flowers 

 are formed on different plants, and only the female was brought 

 to England. In its eighty years' colonisation it has never set 

 seed. 



Such plants seem to have entirely dispensed with the process 

 of sexual reproduction. Whether they can go on indefinitely or 

 whether, for continued vigour, sexual union must take place now 

 and again, we do not know. It is remarkable that so important 

 a food plant as the banana has been multiplied vegetatively for 

 hundreds or even thousands of years. 



Reproduction of Lower Plants 



In the lower plants the reproductive processes are very 

 different from those of the flowering plants. The cone-bearers 

 yew, pine, fir, cedar, juniper produce seeds which are not 

 enclosed in a fruit; the ovules are exposed on the surface of the 

 cone scales. Even the "berry" of the yew is merely a fleshy out- 

 growth only partly covering the seed. 



A degree lower we come, among existing plants, to the ferns 

 and their allies, the club-mosses and horsetails, and these bear 

 no seed. 



On the back of the frond of a fern are to be seen little 

 clusters of spore-cases, each of which contains numerous spores. 

 A spore is a reproductive body consisting of a single cell. When 

 it germinates it grows, not into a fern plant, but into a green 

 blade or prothallus a quarter of an inch long. On the lower 

 side of this prothallus are formed sexual reproductive organs, 

 producing egg-cells and sperms. The sperm is provided with 

 cilia, and lashes itself through the films of surface moisture to 



