Page:An introduction to physiological and systematical botany (1st edition).djvu/97

Rh by the warm days of autumn, when the nights are cold, than in much hotter weather in summer, and this is surely from the same cause as the autumnal flowing of the vegetable sap.

The sap, or lymph, of most plants when collected in the spring as abovementioned, appears to the sight and taste little else than water, but it soon undergoes fermentation and putrefaction. Even that of the vine is scarcely acid, though it can hardly be obtained without some of the secreted juices, which in that plant are extremely acid and astringent. The sap of the sugar maple, Acer saccharinum, has no taste, though according to Du Hamel every 200lb. of it will afford 10lb. of sugar. Probably, as he remarks, it is not collected 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 sensible to us by being collected from a branch introduced into any sufficiently capacious glass vessel,