8 

 Air. 



Winds have great influence on plant forms, and on their dis- 

 tribution. This is "best shown when the winds hlow over great, 

 unbroken stretches of land, or where the force is hroken hy 

 mountains, hills or forests. The influence of the wind can Le 

 seen in regions which have a loose, sandy atrueture. 



Air gives freedom of movement which is so necessary to all 

 plants. Plants cannot have have too much air yet may be injured 

 by some of its const itu^ents. For example, lichens do not usually 

 grow in cities. Smoke is injurious to pines. There is danger 

 in having too little air. Unfavorable conditions for plants ^^jii on 

 high mountains are caused by the rarity of the air. In swamps and 

 pools there is danger of too little 0. There the interchange of 

 gases ia restricted and consequently there is not enoU(i;h ^ 0. 



Too much wind brings a two-fold danger; first it may .tear 

 and break the plant structure, but the greatwr danger lies in 

 causing excessive transpiration. Vegetation has been killed in 

 a single day by a terrific rain storm. Excessive transpiration 

 going beyond power to absorb moisture on account of cold causes 

 low, woody, branching structures. Hx., forests on motuitains, 

 tundras, lichens and mosses of northern regions. The proof that 

 this is caused by dryness and not cold is that in dry, hot countries, 

 plants asume the same forms. V/ind causes the baking of the soil 

 and consequent dryness. 



Leaves and branches are often less developed on the windward 



