Adaptation to Environment. 315 



We have already seen in the chapters on Flowers and 

 Dispersion of Seeds how adaptive modifications have enabled 

 plants to employ the wind in the scattering of pollen and 

 seeds. Plants with these modifications are more apt to 

 abound in windy situations. 



199. The Soil. The different kinds of soils affect the 

 form and structure of plants chiefly in the capacity of soils 

 as water reservoirs. Coarse-grained soils with large inter- 

 stices allow the water to percolate readily through them, 

 and retain but little for the use of plants. Accordingly, 

 plants which inhabit such soils must be modified so as to 

 reduce transpiration and to store water. On the other 

 hand, soils which are fine-grained, and particularly such as 

 are rich in clay or humus, hold large percentages of water, 

 and the plants growing in them are not so apt to possess 

 special devices to restrict transpiration. 



200. Effect of Excess of Salts. If large percentages of 

 soluble salts occur in the soil, they influence the osmotic 

 conditions and render the absorption of water by the roots 

 more difficult ; and it may happen in such cases that even 

 with plenty of water at hand plants may be in danger of 

 drying up. The same sorts of adaptive modifications, 

 designed to retard transpiration, as we find occurring when 

 the soil is dry, are accordingly brought about. 



201. Other Plants and Animals. In the preceding 

 chapters, numerous illustrations of modifications which are 

 adaptive to animals and to other plants have already been 

 discussed. The student may refer to the modifications of 

 leaves to serve as traps, of stems, roots, and leaves for 

 purposes of climbing, of flowers, fruits, and seeds, for 

 the dispersion of pollen and seeds by means of insects and 

 other animals. The discussion of parasitic roots in Chapter 

 III, and of Fungi and Lichens in Chapter XII, has called 



