HEAT. 



151 



emphatically the rcr^ion of the dicotyledons, clothed with 

 transient verdure, and covered by thick non conducting bark. 

 Herbaceous annual plants, seem in their ecoiKxny to have 

 been constituted, in reference to their preservation, during a 

 season in which they could not flourish. The annuals of 

 temper.ito ngions, produce seed, tilted to withstand the vari- 

 ous inlliiencos of a period entirely unsuited to vegetable 

 growtii. They are composed of materials the least affected 

 bv atmospheric influences, being capable of resisting, un- 

 injured, the utmost intensity of cold ; and it is a remarkable 

 fact, that the seoils of tropical annuals which are peculiar to 



tliat rciiion, are much less able to resist the chanires oftem- 



.... 

 perature, and retain their vitality, generally, but for a 



verv sliort time. In the former case, the very continuance 

 of the species depemls on their producing seeds, that will re- 

 tain their vitality, through considerable jieriods, and at the 

 same time, resist the influence of rigorous -climates, while in 

 the lat(er there is not the same necessity for the same pro- 

 visions, and in many instances, at least, these provisions are 

 not made, while ihey are uniformly provided in the other. 



223. Plants, like animals, seem to j)o.<!sess the power in 

 some degree, of preserving a uniform tem]ierature ; whether 

 this is owing in part to the action of vital power, or e.itirely 

 to physic! causes is doubtful. The uniform temperature of 

 the earth, from which ihey derive their food, the non con- 

 duclinir power of the covering, which in a gre.it measure, ex- 

 cludes both the heat of summer and cold of winter, and the evap- 

 oration in hot weather, and its suspension in cold, are causes, 

 perhaps sufficient to account for their uniform temperature. 

 Cases, however, arc mentioned, of plants growing in soil, in 

 the vieinify of hot springs, receiving their food through a 

 medium, but little less than boiling water, and at the same 

 time their temperature was but little affected by these cir- 

 cumstances. 



221. .Although plants may preserve their temperature to a 

 certain extent, yet it is well known, that excess of heat, 

 or cold will destroy theiii The tempeiature they will bear 

 without ifijury, is very diflercnt in different species. While 

 our f )rest trees will bi'ar uninjured, the most intense cold of 

 our winters, others will perish in an atmosphere of tiiirty two 

 degrees, and annuals are destroyed by the first frost of autumn. 

 The manner in which cold operates in the destruction of veg- 

 et; ' ' has of late exeited considerabk; interest. The long 

 pi • ii opinion has been, that the well known jihenomcnon, 

 that water at the moment of its conversion into ice, expands, 



