140 HEAT. 



The equatorial regions are emphatically the regions of monoco- 

 tyledons, destitute of bark, and always in verdure. The tem- 

 perate regions, with the year distinctly marked by the four 

 seasons, is as emphatically the region of the dicotyledons, clothed 

 w^ith transient verdure, and covered by thick non-conducting 

 bark. 



267. Herbaceous annual plants seem in their economy to 

 have been constituted in reference to their preservation during 

 a season in which they could not flourish. The annuals of tem- 

 perate regions produce seed fitted to withstand the various in- 

 tluences of a period entirely unsuited to vegetable growth. 

 They are composed of materials the least atiected by atmospheric 

 influences, being capable of resisting, uninjured, the utmost in- 

 tensity of cold ; and it is a remarkable fact, that the seeds of 

 tropical annuals, which are peculiar to that region, are much 

 less able to resist the changes of temperature, and retain their 

 vitality, generally, but for a very short time. In the former case, 

 the very continuance of the species depends on their producing 

 seeds that will retain their vitality through considerable periods, 

 and at the same time resist the influence of rigorous climates ; 

 while in the latter there is not the same necessity for the same 

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

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



268. Plants, like animals, seem to possess the power in some 

 degree of preserving a uniform temj<erature ; whether this is 

 owing in part to the action of vital power, or entirely to physical 

 causes, is doubtful. The uniform temperature of the earth, from 

 which they derive their food, the non-conducting powder of the 

 covering, which, in a great measure, excludes both the heat of 

 summer and cold of winter, and the evaporation in hot weather, 

 and its suspension in cold, are causes, perhaps, sufficient to ac- 

 count for their uniform temperature. Cases, however, are men- 

 tioned of plants growing in soil, in the vicinity of hot springs, 

 receiving their food through a medium but little less than boil- 

 ing water, and at the same time their temperature was but little 

 aftected by these circumstances. 



269. Although plants may preserve their temperature to a 

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

 will destroy them. The temperature they will bear without in- 

 jury is very diflferent in difi"erent species. While our forest trees 

 will bear uninjured the most intense cold of our winters, others 



"What class of vegetables prevail in equatorial regions? What in tem- 

 perate regions ? — 267. How with annual plants ? — 268. How is the uniform 

 temperature produced ? — 269. Can they resist equally well great degrees ol 

 heat and cold ? 



