10 Professor Gardner on the Influence of the Dem-point 



The tables G and A and D of section the first are of a dif- 

 ferent class ; in these the drying power varies from 10° to 12° 

 and 1 3° ; being about half of the power in the above tables, 

 and representing the air more saturated with watery vapour, 

 and therefore less conducive to evaporation. In these tables 

 we remark an uniform elevation of ■ temperature in all the 

 highly organized parts of the plant; notwithstanding the minus 

 measures of the root from contact with a moist and therefore 

 evaporating soil ; a good illustration, en passant, of the non- 

 conducting nature of living vegetable tissues. 



Not to become diffuse, we perceive in these results, — 

 1st. An uniformity which recommends them to our reason. 

 2ndly. They are in conformity with the experience of man- 

 kind. The effects of moist air on vegetation is known to all, 

 the rapid growth, the vigour of plants, or to speak more scien- 

 tifically, the activity of the chemical and organic actions which 

 maintain life are fully manifest. The result is an increment 

 of temperature in exact proportion to the varying activity of 

 each organ, whether in the respiration of the leaf or the ge- 

 nerative functions of the parts appointed to the reproduction 

 of the species. 



The effects of a drought are no less apparent ; the leaves 

 hang down ; there is an air of listlessness about plants very 

 analogous to the effects of heat upon the human frame, and 

 due to the undue evaporation. 



How firm and succulent is the state of a leaf during moist 

 weather ; how exsiccated and flabby during a dry season ! of 

 this the tobacco planters in Virginia are so well aware, that 

 they esteem moist foggy weather favourable when gathering 

 their crop. It is somewhat curious that these remarks apply 

 to the human family ; the natives of moist countries, as the 

 Netherlands, England, &c, being of fuller habit than those who 

 live in arid regions ; this similitude does not however extend 

 so far as in plants, from the effects of the diseases prevalent in 

 swampy countries. It gives me great pleasure here to recom- 

 mend the paper of Mr. Hopkins in the London and Edin- 

 burgh Philosophical Magazine for February 1839, on Malaria, 

 in which he examines the influence of the hygrometrical state 

 of the air upon animal life. 



At this stage of the investigation it is necessary to meet an 

 objection already urged against the foregoing doctrine, that 

 it levels the principle of life in vegetables to mere chemical 

 action. We do not hold any such view. We simply claim 

 that the sensible caloric generated by plants is the result of 

 internal action; the amount of caloric is also more or less, ac- 

 cording to the activity of the evaporation, the influence of high 



