3 H PLANT LIFE. 



tomed 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 internal structure, 

 also, there are decided differences, particularly in the leaves. 

 (See ^| 167, 438.) The leaves of light plants usually have a 

 thick epidermis, often shiny, with lateral walls straight ; the 

 stomata are frequently confined to the under side and often 

 sunk ; the palisade cells are elongated, sometimes forming 

 two or three layers and occasionally appearing on both faces 

 of the leaf. The shade plants, on the contrary, have a thin 

 epidermis, often containing chlorophyll, with lateral walls 

 often very wavy ; the stomata are produced on both sides of 

 the leaves, and the palisade tissues are poorly developed. 

 Light plants frequently have red cell-sap, especially in the 

 epidermis of smooth plants, and their colors are always 

 deeper, especially in the plants of high latitudes. Shade 

 plants, on the other hand, are usually pale, rarely high- 

 colored. 



426. 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 con- 

 sequently 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 maybe determined. (See ■fl" 186, 263.) The 



