CHAPTER VI. 



INFLUENCE OF EXTERNAL AGENTS ON VEGETATION. 



257. The agents which exercise a decided influence on vege- 

 tation are light, heat, water, and earth. 



The concurrent influence of all these agents, in a greater or 

 less degree, is absolutely required for the perfection of vege- 

 table products ; and according as some of them exist in excess, 

 or in diminished quantity, is the functional operation of the 

 vegetable organs injured or destroyed. Different plants require 

 these agents in very different degrees, and hence the distribution 

 of plants over the face of the globe; some flourishing near the 

 snow-line of the mountains, or near the limits of perpetual snow, 

 north or south, while others can exist only under the influence 

 of tropical heat. Some grow amidst the sandy deserts, others 

 only immersed in water. Some grow in the caverns of the earth, 

 while others must have the direct rays of the sun. Some re- 

 quire a rich soil, others grow suspended in the air. From this 

 adaptation of vegetation to every variety of influence, the earth 

 is covered with verdure ; from the perpetual snows of the 

 mountains or the arctics to the equator, each position giving 

 existence to its appropriate flora. 



Section 1. — Light. 



258t The most obvious effect of light on vegetation is the 

 production of colors, and this it affects by decomposing carbonic 

 acid, and depositing the carbonaceous matter. In most cases, 

 certainly, light is absolutely necessary for the deposition of the 

 green coloring matter, since most plants become perfectly color- 

 less by growing in situations in which they are deprived of light. 

 There are cases, however, in which plants deposit the green chro- 

 mule, when excluded from the light. Green vegetables have 

 been found in caves of the earth, from which the light of day was 

 excluded, and we have seen the cotyledons of the Mustard and 

 the Impatiens balsamina green, when the seeds have germinated 



257. What agents most affect vegetation ? Are they all necessary in some 

 degree ? Do they require them in different degrees? Illustrate it. — 253. 

 What is the most obvious effect of light on vegetation ? Is it universally 

 neoeasary ? W hat examples ? 



