MESOPHYTES. 233 



Tasform of plant parts is directly influenced by light. Plants 

 accustomed to the direct sunlight and those accustomed to 

 shade show profound differences in habit. Light plants are 

 stocky and compact ; their stems are inclined to be woody, 

 the leaves are usually folded or crisped and often set at an 

 acute angle with the direction of the light, and the surfaces 

 are frequently hairy. In contrast, shade plants are slender 

 and sprawling ; their stems often thin and weak ; the leaves 

 flat and smooth and set transverse to the direction of the light- 

 rays, while the surface is slightly, if at all hairy. (3) In inter- 

 nal structure, also, there are decided differences, particularly 

 in the leaves. These differences affect the skin, the number 

 and distribution of the stomata, the form of the cells, and 

 their contents. The sum of the differences distinguishes an 

 upper (illuminated) from an under (shaded) side. 



326. 3. Temperature. — Temperature exercises an im- 

 portant influence upon plants, both upon their aerial and sub- 

 terranean parts. The temperature of the air is really much 

 more important in controlling the adaptations, and conse- 

 quently the geographic distribution, of plants than is light. 

 The reason for this is to be found in the much more unequal 

 distribution of temperature in various regions of the earth's 

 surface. Moreover, temperature affects every vital function 

 of the plant, for each of which a maximum, minimum, and 

 optimum point may be determined. (See ^ 153, 219.) The 

 variations in temperature to which plants are subjected require 

 special adaptations. 



327. (a) Protection against changes of temperature. — 

 These adaptations are to be found in the presence of special 

 substances, such as oils or resins, which reduce the liability 

 of the parts containing them to freeze ; in the reduction of 

 the amount of water in the plant so that less damage results 

 from freezing; and, finally, in the presence of poor con- 

 ductors of heat, such as scale-leaves and hairs in profusion. 



