jio Introduction to Botany. 



another way. Plants of the same species growing at 

 progressively higher elevations on the mountains may 

 produce in the cells of the epidermis and deeper-lying 

 tissues more and more of a red coloring matter, which 

 evidently serves the purpose of cutting off a part of the 

 light, whose intensity increases with the altitude to such 

 an extent as to prove destructive to the chlorophyll. 

 When plants, particularly those of prostrate habit, grow 

 in shady places, the red color is apt to occur in the lower 

 epidermis, and appears to serve the purpose of arresting 

 the light which has passed the upper tissues, reflecting 

 part and transforming part into energy of heat. Intense 

 illumination seems also to give rise to the production of 

 hairs, scales, etc., that reflect part of the light, and also 

 reduce transpiration, which the intense illumination tends 

 to accelerate. 



194. Temperature. — Variations in temperature may exert 

 a very marked influence on the processes of nutrition and 

 growth, and therefore on the size of plants ; but no visible 

 adaptive changes in form and structure are known to be 

 brought about solely by exposure to different temperatures. 

 On the contrary it frequently happens that the plants of 

 the polar regions or cold mountain peaks have the same 

 general appearance and construction as those of hot deserts. 

 But although different degrees of heat may not give rise to 

 adaptive changes in form, they do produce very radical 

 adaptive alterations in the qualities of the protoplasts, for 

 it is well known that plants of tropical and temperate 

 regions would quickly succumb to the low temperatures 

 which plants of polar regions are known to endure. 



195. Resistance to Cold. — The plants of the Siberian for- 

 ests, for example, withstand temperatures as low as 60° C. 

 (76° F.) below zero. Kjellman, the botanist of the Vega 



