100 



their leaves freely to the sun. Not being compelled to con- 

 centrate its efforts, in securing a scanty supi)ly of one 

 beneficial influence, all its proportions are absolute and 

 universal, not relative and particular. In such circumstan- 

 ces, therefore, it may be considered as in a full and natural 

 state of perfection. 



Another condition of vegetable life appears to be an ade- 

 quate 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 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 ; and their chemical 

 composition is such, that they do not congeal, vmless the 

 cold be of the severest sort, and many degrees below the 

 freezing point of water. Some caloric, however, they pro- 

 bably possess, otherwise they would be killed in very hard 

 weather, or rather, on the too sudden return of heat.* 



Of the above accidents nature can modify the former, by 

 accommodating different species of plants to different lati- 

 tudes 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 fad. to n cortnin extent alleviated, by various pro- 



"NOTF IV. 



