CHAP. I. VITAL PROPERTIES AND STIMULANTS. 1 "3 



which different plants are capable of existing are widely 

 apart. Some flourish within the influence of hot springs, 

 where they are stated to be constantly exposed to a tem- 

 perature of 62 R., or 17H F., and even to 80 R., 

 which is equivalent to 212 F. ; whilst the oak sustains 

 the rigours of a winter in latitudes where the thermo- 

 meter falls to -25 R., or -24-i F., and the birch will 

 resist a cold of -36 R., or -49 F. The latter is well 

 protected against the effects of extreme cold by the man- 

 ner in which its trunk is defended with several loose coats 

 of epidermis. The chief protection, however, against 

 the sap freezing in the trunks of trees, is the circum- 

 stance of its being contained in extremely minute ve- 

 sicles and capillary vessels ; for it has been shown that 

 water will resist a temperature of -? R. or l6-^- F. 

 under similar circumstances ; and all viscid fluids are 

 still more difficult to freeze than water. Whenever 

 the sap does freeze, it produces the effect technically 

 termed " shakes " in timber trees, which consists in a 

 tendency in the separate layers of wood to disunite. 



(158.) Internal Temperature. In animals, the 

 function of respiration is the means by which caloric is 

 set free, for the purpose of maintaining the temperature 

 of their bodies at a sufficient elevation to protect them 

 against the influence of cold, and perspiration cools them 

 when they are exposed to excessive heat. As vegetables 

 perform two functions of a similar kind, we might per- 

 haps be led to expect that the influence of similar 

 effects would regulate their internal temperature. But, 

 if such be the fact, the results are on too minute a 

 scale to be rendered sensible by our instruments ; and 

 in the winter, when these functions nearly cease, we 

 cannot suppose that they operate at all in resisting any 

 atmospheric changes which might be injurious to vege- 

 tation. Still it has been observed as a general law, that 

 the temperature of a tree is higher between autumn and 

 spring than the average temperature of the air, and 

 that it is lower between spring and autumn. But 

 there are physical causes which seem to be sufficient 



