CHYMISTRY APPLIED TO AGRICULTURE. 11 



dergoing the change, and thus produce heat. In combus- 

 tion, where oxygen is the principal agent, there is a 

 constant disengagement of caloric, because that gas, in 

 general, forms solid or liquid compounds with combustible 

 substances ; and it gives out a portion of the caloric which 

 preserved it in its gaseous state. 



These principles established, we can easily explain a 

 part of the effects produced upon vegetation by the varia- 

 tions of temperature. 



The changes of temperature, experienced by the atmo- 

 sphere in the course of a year, are so great, as to cause 

 some liquids to pass alternately either to the solid or aeri- 

 form state, and some solid bodies to become liquid. The 

 natural effect of heat upon these bodies is, by dilating 

 them, to weaken the force of cohesion which unites their 

 molecules, and, by facilitating the action of chymical 

 affinity, to enable them to enter into combination with 

 foreign bodies. Thus heat renders the juices of plants 

 more fluid, and facilitates their circulation through the 

 cells and capillary vessels ; and, by giving activity to the 

 suckers of roots, enables them to draw from the earth the 

 juices necessary for their nourishment. 



Above a certain temperature, heat, by promoting evapo- 

 ration, causes the juices of plants to become thickened 

 and dried in their organs, and thus vegetation is arrested, 

 and life suspended. This effect always takes place during 

 great heats, when neither rain, dew, nor irrigation can 

 sufficiently repair the loss occasioned by evaporation. 

 This effect would be more frequent, if provident Na- 

 ture did not employ means to moderate the action of heat. 



The first of these means is the transpiration of the vege- 

 tables themselves, which cannot take place without carry- 

 ing off a large portion of heat, and thus preserving the 

 transpiring body at a temperature below that of the air. 

 The second means is found in the organization of leaves, 

 which are the only parts of a plant where transpiration 

 takes place. That surface of leaves which is exposed to 

 the direct rays of the sun is covered by a thick epidermis, 

 which resists the calorific rays. In herbaceous plants, as 

 in the stalks of grasses, this covering is composed princi- 

 pally of silex. In other plants it is analogous to resin, 

 wax, gum, or honey ; whilst the epidermis, which covers 

 the opposite sides of the leaves, is fine and transparent. 



