“Another condition of vegetable life appears to be an adequate degree of Heat. Within a certain range of temperature, vegetation is positively promoted: Below, or above a certain point (the degree differing in different species of plants), vegetation is positively checked. To speak only of the latter case, which is briefly expressed by the term {258} Cold, it is either produced by absolute lowness of temperature, or, in particular circumstances, by the generation of cold, through the effect of wind, and consequent evaporation from a moist surface; for trees, in themselves, have but little self-generated heat, above the surrounding temperature. Some they certainly possess, otherwise they would be killed during severe frosts. Of the above accidents nature can modify the former, by accommodating different species of plants to different latitudes and elevations: Against the latter she adopts the plan of affording suitable protection to the individual. In the interior of woods, where the free current of air is intercepted, where stillness and serenity are maintained, and where each tree affords shelter, more or less, to every other, nature has little need to generate the provisions necessary to mitigate the injurious effects of evaporation. But, in open exposures, and in the case of isolated trees, this effect must be assuaged, and is, in fact, to a certain extent alleviated, by various provisions or properties, bestowed upon the tree itself. In the first place, a thicker and closer ramification of the sides and top is supplied, and a more abundant spray towards the stormy quarter, thereby furnishing a kind of clothing of leaves, in order to protect from cold both the {259} ascending and the descending sap-vessels: And, secondly, a greater induration of the epidermis, and thickness of the cortical layers of the bark are provided; which, forming a bad conductor of heat, act as a still more effectual defence to the stem, by preventing the immediate and powerful application of cold, through the sudden subtraction of caloric, from the proper vessels of the inner bark.