ECOLOGY 58 



or spherical, much reduce the area of surface which can 

 evaporate in proportion to their contents ; the rolled- 

 up leaves not only save the exposure of both surfaces 

 at once, but, in general, their pores are only on the side 

 which is rolled inmost, and so evaporation, or transpira- 

 tion, takes place into the nearly closed cavity made by 

 the rolled leaf instead of into the open air ; while the 

 woolly covering of hairs prevents the air currents 

 sweeping over an unprotected surface and tending to 

 dry it, for the felt of hairs helps to keep the air stagnant 

 over the pores and thus to reduce the amount of tran- 

 spiration. Reference must be made to the numerous 

 instances of such adaptations described in nearly every 

 book on botany. 



An interesting point to notice is the tendency that 

 several swamp and salt marsh plants^ show to develop 

 some of the characteristics of desert vegetation. This 

 is to be explained by the fact that the water, which is 

 present in abundance in a physical sense in swamps or 

 salt marshes, is wanting in a physiological sense, because 

 water that is heavily charged with humic acid or with 

 mineral salts is of very little use to the plant. As we 

 mentioned in the chapter on physiology the roots 

 absorb the water in the soil by a process of osmosis. 

 Now, in this chemical process the majority of the com- 

 pounds dissolved in the water enter with it, but if the 

 solution is too strong then more salts enter in solution 

 than the cells can use up, and the cells get clogged and 

 poisoned. Hence the entry of the water must be re- 

 stricted, and hence the surface transpiration must not 

 be too great, and the plant is as badly off for water as 

 if it were living in a region where there is very little in 

 the soil. 



In considering plant communities we have not only 



