HOW PLANTS LIVE AND WORK. 339 



their family, still, like the Ferns and Club-mosses, keep up the 

 ancient traditions of their race and exhibit the characteristic 

 alternation of generations. The plant we know as the Horsetail 

 is the non-sexual generation and it produces a sexual Prothallus, 

 which is amazingly like that of the Ferns. That two plants whose 

 non-sexual generations are so widely different should have practi- 

 cally the same form of Prothallus is very curious and suggestive, 

 and probably indicates that the Prothallus is the primitive form, 

 and that the spore-bearing plant with which we are familiar has 

 become modified to suit its conditions in order that it may be 

 better able to scatter the spores. 



In some of the lower Liverworts, the obvious plant itself is 

 very much like the Prothallus of a Fern, and the spore-bearing 

 generation, instead of becoming big and strong like a Fern, remains 

 as an insignificant and partly parasitic form upon the sexual 

 generation. In fact, in all the " moss-like " plants, including the 

 true Mosses and the Liverworts, the spore-bearing generation, 

 instead of growing a stem and leaves like a Fern, is simply an 

 unostentatious " fruit " upon the prominent and often highly 

 developed stage which corresponds to the lowly Prothallus of the 

 Fern and Horsetail. There is thus between the Vascular Crypto- 

 gams and the moss-like plants a great gulf fixed. Great as is the 

 gulf, it is as nothing in comparison with the enormous difference 

 which exists between the elaborate and highly specialised sea-weed 

 which gives its name to the Sargasso Sea, and the minute plants 

 which form the green, powdery deposits on old palings and walls. 

 Yet these and an enormous number of intermediate forms of 

 plants are grouped by botanists in the same sub-kingdom. It 

 would be quite impossible, in a brief and simple account like the 

 present, to enter into detail concerning the wonderful beauty and 

 marvellous life histories of many of these forms. The majority 

 of them are invisible to the naked eye, and since they are unknown 

 except to botanists, they possess only technical names which would 

 be meaningless to the general reader. They all possess chloro- 

 phyll, and can consequently build up food for themselves. Many 

 of them form spores, which swim actively in the water ; and some 

 spores are capable of distinguishing between light and darkness, 

 and of regulating their movements accordingly. And the lower 



