52 OF THE SAP, AND INSENSIBLE PERSPIRATION. 



cording to Du Hamel every 200 ft. of it will afford 

 10 ft. of sugar. Probably, as he remarks, it is not col- 

 lected without an admixture of secreted fluids. 



As soon as the leaves expand, insensible perspiration 

 takes place very copiously, chiefly from those organs, 

 but also in some degree from the bark of the young 

 stem or branches. The liquor perspired becomes sen- 

 sible to us by being collected from a branch introduced 

 into any sufficiently capacious glass vessel, and proves, 

 for the most part, a clear watery liquor like the sap, 

 and subject to similar chemical changes. It is observed 

 to be uniform in all plants, or nearly so, as well as the 

 sap, except where odorous secretions transude along 

 with it. Still there must be a very essential difference 

 Between the original sap of any plant and its perspira- 

 tion, the latter no longer retaining the rudiments of 

 those fine secretions which are elaborated from the 

 former ; but that difference eludes our senses as well 

 as our chemistry. The perspiration of some plants is 

 prodigiously great. The large Annual Sunflower, Hdi- 

 anthus annuus, Gerarde Emac. 7.5 1 . f. 1, according 

 to Dr. Hales, perspires about 1.7 times as fast as the 

 ordinary insensible perspiration of the human skin. 

 But of all plants upon record I think the Cornelian 

 Cherry, Cornus mascula, is most excessive in this 

 respect. The quantity of fluid which'evaporates from 

 its leaves in the course of twenty-four hours, is said to 

 be nearly equal to twice the weight of the whole shrub. 

 Du Hamel Phys. des Arbres, v. 1. 145. 



