14 Prof. Gardner on the Influence of the Dew-point on Vegetables. 



cific temperature that varies within one-third plus or minus 

 the atmospheric temperature, which may be 90° Fahr. at noon, 

 and 40° in the evening. The real cause of the elevation or 

 depression measured, is to be found in the more or less per- 

 fect saturation of the atmosphere in which the experiments 

 were conducted. There is, however, a great difference be- 

 tween the amount of heat measured by M. Dutrochet and 

 myself; but whatever may be the cause of the discrepancy, the 

 measures given in the tables are certainly free from error, 

 since most of them were authenticated by the simultaneous 

 examination of my friends at Hampden Sidney College. 



We are much more concerned by the apparent anomalies 

 exhibited by Nature. Why are not all plants destroyed by 

 frost? Why do not tubers, bulbs, &c. perish during winter? 

 For if there be no specific heat in these organized substances, 

 their fluids should freeze and thereby produce disorganiza- 

 tion. In reply to this we remark, that the fluids of vegetables 

 congeal at temperatures below the freezing point of water in 

 consequence of the presence of mucilage and acids, &c. Again, 

 the degree of succulence of the plant and strength of the 

 tissues, as well as their non-conducting nature, must not be 

 lost sight of. It is remarkable that all northern evergreens 

 have more or less coriaceous leaves. The vegetation of coun- 

 tries invaded by cold is hardier than that found in the tropics; 

 in the former localities the majority of plants are annuals or 

 perennials, or trees which cast their leaves ; whilst in the south 

 evergreen trees abound which are incapable of enduring ex- 

 posure to one frost. Our trees are often found with their sap 

 frozen without the texture being destroyed ; and in the Annates 

 de Chem. et de Phys., torn. xv. p. 84, there is an account of a 

 parcel of young trees which were kept in a frozen state for 

 twenty-one months and yet finally vegetated when gradually 

 thawed and planted out, showing conclusively that the woody 

 fibre resisted the disruptive force of the expanding water when 

 in the act of freezing. The non-conducting nature of the 

 bark and wood is another powerful protection ; we witnessed 

 a poplar tree cut down in the depth of winter ; on the northern 

 side of the trunk the wood was quite dry and the sap probably 

 frozen, whilst on the southern exposure the sap was fluid : 

 this fact proves the necessity of paying every attention to the 

 exposure of trees which are transplanted in the winter, espe- 

 cially evergreens. 



Many roots, tubers, bulbs, &c. may be exposed with appa- 

 rent impunity during winter, but if we examine the conditions 

 necessary to secure them, it is found that they must be either 

 covered with soil or are naturally of a dry and amylaceous 



