EVOLUTION OF THE LAND PLANT 103 



The altered conditions of their life can now be seen to 

 lead to a very great change in the life history of plants. 

 The aquatic forms with their comparatively simple body 

 and the process of fertilisation depending on the power 

 of the sperm to swim to the ovum proved exceedingly 

 unsuitable for life on land, and very soon a great develop- 

 ment of the sporocarp began, and it gradually assumed 

 the form of a self-supporting plant the sporophyte. 

 From this point upward the tendency of evolution was 

 to diminish the plant which bore the ova and sperms till 

 there was hardly anything of it except these repro- 

 ductive cells, and at the same time to make the sporo- 

 phyte more and more important, till it became far the 

 most highly developed, and of infinitely larger size than 

 what was left to represent the original ancestor. 



We find this change well established in the ferns and 

 the plants which are allied to them. The ova and 

 sperms are produced on a thin plate-like body of about 

 half an inch in diameter. This lies upon the soil and is 

 attached to it by rhizoids. The antheridia and arche- 

 gonia are borne upon its lower surface and fertilisation 

 is accomplished by free-swimming sperms. The plant 

 is known as the prothallus ; it is made up of cells which 

 are much alike throughout. The sporocarp arising from 

 it has been replaced by a large plant, showing root, stem, 

 and leaves. We do not know the stages by which so 

 large and well-organised a structure has been developed 

 from the original sporocarp, but it represents the latter 

 in its place in the life history of the fern. So important 

 has it become, and so great in comparison with the 

 prothallus, that it has come to be called the " plant," 

 leaving the ovum-bearing plant to be regarded as the 

 " prothallus of the fern." 



We see that with the establishment of terrestrial 

 habit, the life history of the plants thus became quite 

 revolutionised. The large tree of the land flora does 



