142 PERSPIRATION OF LEAVES. 



wax, or any other fit substance, to prevent the effusion 

 of moisture in that direction. In a very short time the 

 leaves droop, wither and are dried up. If the same 

 branch, partly faded, though not dead, be placed in a 

 very damp cellar, or immersed in water, the leaves 

 revive, by which their power of absorption is also 

 proved. Hence the use of a tin box to travelling bo- 

 tanists, for the purpose of restraining the evaporation 

 of plants, and ,so preserving them fresh for some days 

 till they can be examined, as well as of reviving faded 

 plants, if the inside of the box be moistened before 

 they are shut up in it. 



Dr. Hales found that a plant of the Great Annual 

 Sunflower, Helianthus annuus, lost 1 IB. 1 4 oz. weight 

 in the cCurse of twelve hours in a hot dry day. In a 

 dry night it lost about 3 oz. ; in a moist night scarcely 

 any alteration was observable, but in a rainy night it 

 gained 2 or three oz. The surface of the plant compared 

 with that of its roots was, as nearly asx:ould be calcu- 

 lated, in the proportion of five to two ; therefore the 

 roots must have imbibed moisture from the earth of 

 the pot in which the plant grew, and which was all 

 previously weighed, in the same proportion of five to 

 two, otherwise the leaves would have faded. The same 

 experiment was made on the Vine, the Cabbage, c., 

 with various results as to the exact degree of perspira- 

 tion, but all proving it to be considerable. Evergreens 

 are found to perspire much less than other shrubs. 



The state of the atmosphere has a great effect on the 



