CHAPTER XII. 



SUB-KINGDOM V. 

 PTERIDOPHYTES. 



IF we compare the structure of the sporogonium of a moss 

 or liverwort with the plant bearing the sexual organs, we find 

 that its tissues are better differentiated, and that it is on the 

 whole a more complex structure than the plant that bears it. 

 It, however, remains attached to the parent plant, deriving its 

 nourishment in part through the "foot" by means of which 

 it is attached to the plant. 



In the Pteridophytes, however, we find that the sporogo- 

 nium becomes very much more developed, and finally becomes 

 entirely detached from the sexual plant, developing in most 

 cases roots that fasten it to the ground, after which it may live 

 for many years, and reach a very large size. 



The sexual plant, which is here called the " prothallium," is of 

 very simple structure, resembling the lower liverworts usually, 

 and never reaches more than about a centimetre in diameter, 

 and is often much smaller than this. 



The common ferns are the types of the sub-kingdom, and 

 a careful study of any of these will illustrate the principal 

 peculiarities of the group. The whole plant, as we know it, 

 is really nothing but the sporogonium, originating from the 

 egg cell in exactly the same way as the moss sporogonium, and 

 like it gives rise to spores which are formed upon the leaves. 



The spores may be collected by placing the spore -bearing 

 leaves on sheets of paper and letting them dry, when the ripe 



