The Origin of Land Plants 83 



Resting Spores of Green Algae. Most of the 

 green algae at the close of their active growing 

 period produce some form of resting spore, thick- 

 walled cells which can survive complete desiccation, 

 and are thus able to carry the plant over periods of 

 drought. While a small number of algae, like the 

 Botrydium already referred to, may assume a more 

 or less complete aerial habit, this is exceptional, and 

 we must look to the next branch, or sub-kingdom, 

 of plants, comprising the mosses and ferns, for the 

 first green plants which may be considered to be 

 normally of terrestrial habit. 



The First Terrestrial Plants. The first invasion 

 of the land by the algal ancestors of the higher 

 plants must be regarded as a most momentous 

 event in the history of the vegetable kingdom. 

 The much greater range of conditions on land 

 affords far greater possibilities for structural 

 variation, and this is amply shown in the future 

 history of the plant kingdom. The higher plants 

 are mainly organisms adapted to life in the air, 

 and show a complexity and variety of struc- 

 ture far surpassing that of the largest and most 

 specialized of the seaweeds, which seem to have at- 

 tained the limits of structure possible within the 

 range of their strictly aquatic environment. The 

 mosses and ferns, as we shall see, show unmistakable 

 evidence of their derivation from aquatic ancestors, 

 and indeed all of these forms may be considered to 

 be amphibious, as the development of certain phases 



