THE AGENTS WHICH AFJTECT PLANTS. 



9 



colours forms green, as blue paint mixed with yellow 

 produces green. This will account for the yellow colour 

 of leaves when the pulp is deficient. 



II. THE AGENTS WHICH AFFECT PLANTS. 

 l.—Heat. 



Before ever sap can be extracted from the soil^ or set in 

 motion afterwards, it must be acted upon by heat, which 

 is the prime agent in promoting the growth of plants. It 

 is present both in the soil and in the air, and is every- 

 where diffused. In proportion to its prevalence, (other 

 conditions being available,) the growth of plants will be 

 either rapid or extensive, or the contrary. This accounts 

 for the comparative dormancy of plants in winter or cold 

 weather. The susi)ension of the flow of sap at the begin- 

 ning of winter is erroneously ascribed to the descent of the 

 sap at that season, when, in fact, it is solely occasioned by 

 the absence of a sufficient degree of light and heat. If 

 these and moisture were duly present, perpetual growth 

 would be the result, as it is, except during the dry season, 

 in tropical climates. 



Heat is distributed pretty equally among all things on 

 the earth's surface, by a process somewhat similar to that 

 of water always finding its level, and which is termed 

 radiation. Thus, it will invariably pass from a warm sub- 

 stance to one near it which is colder, and all the more 

 rapidly if the two things are porous and in contact. The 

 warm ground, for instance, will give off its heat into the 

 cold air, till the heat of the ground and that of the air 

 become equalised; but the air will not communicate its 

 heat so readily to the ground, as it is a property of heat 

 to be continually ascending, and passing off into space. 



As plants derive their food mainly from the soil, its 

 heat should be in some measure correspondent to that of 

 the atmosphere, or they will increase in length, but not in 

 strength. This is one reason why cold wet soils are gene- 

 rally unsuitable. The roots do not grow in proportion to 

 the branches and leaves. 



Cold is merely a state in which, by the process of radia- 

 tion, heat is absent. Plants always possess a certain 



