42 Plant Life and Evolution 



Changes Due to Terrestrial Habit. The transla- 

 tion to land involves at once a complete readjust- 

 ment of the plant to its water relation. The ex- 

 posed surfaces must be protected against excessive 

 evaporation, and this, of course, implies a diminu- 

 tion of the power of absorbing water from with- 

 out. While some land plants, like certain Cali- 

 fornian mosses and ferns, and the resurrection plant 

 (Selaginclla lepidophylla) of Arizona, can absorb 

 water through their leaves, thus behaving like algae, 

 this is not possible in most land plants, which pro- 

 cure their water mainly through the root system. 

 The need for rapid distribution of water through the 

 plant is met by the development of an elaborate 

 " fibre-vascular " system of tissue in the higher 

 plants. 



The ability of the plant to regulate the loss 

 of water through evaporation may be readily dem- 

 onstrated by comparing plants of the same species 

 grown under different conditions. Where moisture 

 is abundant and the need of economy small, the 

 rank-growing plant has thick stems and large 

 thin leaves with a poorly developed cuticle. This 

 same plant grown in full sunshine, with a limited 

 water supply, is comparatively small, and not only 

 is there a smaller surface exposed to evaporation, 

 but the surface tissues are very much less per- 

 meable. These latter conditions are most pro- 

 nounced in plants of hot dry regions where the 

 problem of existence is concerned first of all with 



