8 Professor Gardner on the Influence of the Dew-point 



keep down the heat of the root, even when chemical action is 

 taking place most actively in its structure. 



We are therefore justified in asserting that vegetables (so Jar 

 as annuals and perennials) possess no specific heat similar to 

 that belonging to mammals. Sec, but that their temperature varies 

 'with the atmosphere within certain limits. 



§ 2. That 'the variations plus or minus the atmospheric tempera- 

 ture are partly owing to the state of the dew-point, fyc. (p.l.) 

 It is well known that evaporation cannot take place from any 

 surface unless the temperature and dew-point differ ; for as 

 a given bulk of air is only capable of retaining a certain 

 amount of watery vapour in solution at a known tempera- 

 ture, it follows, that if the dew-point indicates that amount of 

 saturation, all evaporation must cease so long as these condi- 

 tions are maintained. It is also well known, that the heat 

 produced by chemical and vital actions taking place in the 

 highest animals is antagonized by evaporation from the skin 

 and lungs, the tendency of which is to produce coldness. We 

 have here therefore a source of heat and its opposite which 

 likewise exists in plants, with this difference, that whilst the 

 former power is considerably lessened, the latter is increased 

 in consequence of the extensive surface from which evapora- 

 tion takes place. 



But the rapidity of evaporation is dependent upon several 

 circumstances, as the amount of drying power, velocity of the 

 wind, extent of surface, &c. ; of these the first is the most im- 

 portant and easiest of examination. To show its influence, 

 we introduce three other tables, selected as illustrating the 

 influence of the amount of drying power most extensively. 



Table E. 

 June 12th. Arum Walteri; soil extremely wet, and conse- 

 quently adhering less firmly than in the previous cases. Ther- 

 mometer 85°. Dew-point 60°. Drying power 25°. Clear. 



