102 BIOLOGY OF PLANTS. 



pand all bodies, gaseous, liquid and solid ; and a third is, its in- 

 fluence on affinity. 



1. Its influence on affinity is due to its solvent and volati- 

 lizing properties. By its accumulation in bodies, it destroys 

 cohesion and dissolves or melts them, and brings their parti- 

 cles into contact, so as to produce chemical combinations ; or 

 else it volatilizes the bodies, and removes the particles from the 

 influence of each other's attractions. In these ways it aids or 

 retards the decompositions and recompositions in the soil, and 

 becomes a powerful agent in preparing the proper food of 

 plants. 



2. The effect of heat in the spring and summer is to ex- 

 pand the tubes or vessels of vegetables, in which the sap cir- 

 culates, and also, if not too great, to render the juices more 

 fluid. It thus promotes a more easy introduction of sub- 

 stances into the roots, and a more rapid circulation through 

 the stems, branches and leaves. 



But if the heat is too great, which often happens during 

 dry seasons, when neither rain nor dew is sufficient to coun- 

 teract its effects, the juices become thickened, and the vessels 

 contracted ; the plant shrivels up, and often wilts and dies. 

 This effect would be produced more frequently, were it not, 



3. For the tendency of heat to pass into an insensible state 

 in the process of evaporation of water from the soil, and from 

 the plant itself, and were not the plant otherwise protected, 

 from extremes of temperature. 



The fluids, which pass into the plant, are some of them 

 converted into solids by assimilation. This process increases 

 the temperature of the plant, by rendering hisc7isihlc caloric 

 sensible, and is a means of heat, when sufficient quantities 

 from without are not supplied. But the greater part of the 

 water, which enters the root, is thrown out or transpired by 

 the leaves, and other green parts. The moisture thus thrown 

 out upon the outer surface, by passing into vapor, absorbs the 

 sensible and external heat. The higher the temperature, the 



