306 Introduction to Botany. 



or boisterous, warm or cold, humid or dry, foggy or clear, 

 and rare or dense, depending on elevation. 



Also in considering the environmental conditions to 

 which plants must become adapted we must take account 

 of the maximum, minimum, and average temperatures for 

 the year ; maximum, minimum, and average rainfall ; time 

 of greatest rainfall, whether during the growing season or 

 during the period of vegetative rest ; the direction of the 

 prevailing winds and their maximum and average forces ; 

 and of the intensity and quality of the light as it varies 

 from low to high elevations, and from the equator to the 

 poles. Indeed, the conditions appear to be too numerous 

 and varied for us to assign to each its proper value ; but 

 plants feel the influence of each factor and accommodate 

 themselves to it. We shall here discuss simply the main 

 factors with which plants have to deal, namely, water 

 supply, light, temperature, atmosphere, soil, and relation 

 to animals and to other plants. 



187. Water Supply. Variations in the water supply 

 are more effective in requiring adaptive changes in the 

 forms and construction of plants than variations in any 

 other factors (see pages 37 and 1 10, for a discussion of the 

 value of water to plants). When there is plenty of water 

 available in the soil, but not to the extent of saturation, we 

 find plants much branched above and below ground, with 

 a large expanse of leaf surface; the stomata occur fre- 

 quently on both sides of the leaf, but the greater number 

 usually occur on the under side. The great majority of 

 plants in regions having a temperate climate and abundant 

 rainfall are of this character. 



188. Effect of Scarcity of Water. A scarcity of water 

 necessitates certain profound changes from this type which 

 are designed to reduce transpiration ; for if plants are un- 



