28 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



with extreme temperatures, and in arctic and alpine situations are due 

 to cold, but in deserts are called forth by great dryness, and evaporation 

 arising from intense heat. The shoots become short and curved, in the 

 former places because the warmth requisite for growth is wanting, and in 

 the latter site because heat robs them of moisture.^ 



From what has been said it follows that various structural features 

 apparently must be interpreted as due to the action of heat upon plants. 

 Attention will be directed later in this work to the great importance of 

 the temperature of the air in relation to atmospheric humidity and to 

 transpiration ; these exercise great influence upon plant-form. 



The distribution and habitats of species in their main features (vege- 

 tative zones of the earth, vegetative regions of mountains) are determined 

 by heat. In terrestrial species of wide geographical distribution the 

 difference between the maximum and minimum temperatures is as a rule 

 especially wide (this is not true of aquatic species). But, above all, heat 

 exerts an influence upon the habitat, economy, and struggles of plant- 

 communities. It partly determines the distinction between the climates 

 and vegetation of the coasts and interiors of countries ; this is most 

 clearly shown in arctic countries, where the feeble vegetation of the cold 

 coasts contrasts with that of the interior, which is relatively rich in 

 species and individuals and displays more vigorous individuals.- 



Arctic countries also exhibit great contrasts between the feebler 

 vegetation of lowlands and the richer, more luxuriant vegetation of sunny 

 mountain-slopes ; for the angle of incidence of the sun's rays is far more 

 acute in the plains than in the declivities. If steep mountains should 

 occur near the Poles they certainly must have a relatively rich vegetation. 

 The angle of inclination and the exposition of mountain-slopes obviously 

 influence the result, since the soil and consequently the air will be heated 

 to different degrees when these differ. But as these and other conditions 

 depend intimately on the temperature of the soil, their treatment is rele- 

 gated to Chapter XIII. That the contour of the earth's surface may 

 influence details in the geographical distribution of plants may often be 

 seen in situations where, on calm frosty nights, cold air remains suspended 

 over depressions and valleys and causes the plants to be frost-bitten. 



CHAPTER VII. ATMOSPHERIC HUMIDITY AND 



PRECIPITATIONS 



The ecological importance of water to the plant is fundamental, and 

 almost surpasses that of light or heat. Without water, vital activity is 

 possible neither to plant nor animal. Its significance to the plant in a con- 

 dition of full vital activity is as follows : 



1. As imbibition-water it is necessarily present in all protoplasm and all 

 cell- walls. 



2. As cell-sap it occurs in vacuoles and plays a part in turgidity and 

 normal growth. 



3. As a nutritive substance it is directly assimilated. 



^ Further details are given in Chapter VIII, deaUng with the effects of wind. 

 ^ In regard to the influence of light see Chap. V. 



