90 Plant Life and Evolution 



to enable them to maintain a truly upright position. 

 The larger species are either prostrate, as we have 

 seen in many large liverworts, or the upright posi- 

 tion is maintained by the shoots being densely 

 crowded and thus affording mutual support. It 

 must be remembered that the gametophyte of the 

 archegoniates is the transformed progeny of some 

 strictly aquatic plant, and it is not unlikely that 

 there are limits beyond which such a type cannot 

 progress. So far as we know, the higher mosses 

 represent the extreme development on land of these 

 originally aquatic organisms, and they cannot be 

 said to have solved very satisfactorily the problem 

 of the development of a plant type perfectly adapted 

 to life on the land. 



Evolution of the Sporophyte. The further evo- 

 lution of the plant kingdom is mainly bound up with 

 the neutral generation or sporophyte. The origin 

 of this is to be looked for in the zygote, or resting 

 spore, so commonly developed in the green algae, 

 as the last phase of their life history. This zygote 

 may be said to represent the terrestrial phase of the 

 alga, as it is fitted to survive drought, and thus to 

 carry the plant over from one growing period to 

 another. The fact that the zygote, which is the 

 morphological equivalent of the sporophyte of the 

 mosses and ferns, is from the very first a structure 

 fitted for existence outside the water, must be borne 

 in mind in following out the further history of the 

 evolution of the higher plants. 



