206 



PHYSICAL ENVIRONMENT 



and are heaviest on the leeward side. Figs. 351, 352. Ob- 

 serve this fact in orchards in windy regions, and note that 

 the most unsymraetrical trees are those on the exposed side 



of the plantation. 



344. Trees often 

 lean away from 

 the prevailing 

 winds. Fig. 353. 

 The tips of the 

 branches of ex- 

 posed trees usuall}' 

 indicate whether 

 there are strong 

 prevailing winds. 

 Fig. 354. Observe 

 trees in pastures 

 and along road- 

 sides, particularly 

 in high places and 

 Avithin a few miles 

 of exposed shores. 

 Note the tip -top 

 spraj' of hemlock 

 trees. 

 345. PLANTS ARE PROFOUNDLY INFLUENCED BY SOIL.— 

 The food supply varies with the kind of soil; and the 

 food supply determines to a large extent the character 

 of the individual plant. On i)oor soils plants are small; 

 on rich soils they are large. The difference between pojjr 

 and good yields of wheat, or any other crop, is largely a 

 question of soil. The farmer reinforces his poor soils by 

 the addition of fertilizers, in order to make his plants vary 

 into larger or more productive individuals. 



34G. The moisture-content of the soil exerts a marked 

 influence on plants. We have found (154) that a large 



354. A tree tliat sliows wlii 



Oklahoma. 



;iy t)io wiiid blows. 



