236 OUTLINES OF PLANT LIFE. 



the form of the plant is strikingly shown in the occurrence 

 of certain species of a genus only upon soils containing lime, 

 while others of the same genus are found only in soils free 

 from lime. When the local distribution of corresponding 

 species of the same genus within the same region is deter- 

 mined by the presence or absence of lime in the soil, com- 

 parison of them indicates the general effect of lime salts upon 

 the plant. Plants growing upon lime are usually stronger 

 and more densely hairy, often hoary, while those on other 

 soils are smooth or furnished with glandular hairs. The 

 lime-loving plants have bluish-green leaves, as contrasted 

 with the grass-green. Their leaves are also more numerous 

 and more deeply branched, the flowers larger and their colors 

 duller and paler. 



333. Summary. — Mesophytes have a moderate water sup- 

 ply. Arbitrarily selected as the norm, their adaptations are 

 nevertheless as numerous and important as those of other 

 plants, but less striking only because they are familiar to the 

 eye. Thus they adjust themselves in form and structure to 

 the wind, the light, temperature, moisture and rainfall, and 

 the soil. The light influences the rate of growth and de- 

 velopment, and especially internal structure, often inducing 

 a two-sided structure, as in leaves. Changes in temperature 

 call out protective adaptation against sudden changes, and 

 a dormant period (during winter), and also affect the form 

 of plants, as do moisture of the air, rain and snow. The 

 substances in the soil may also modify the form of a plant. 



