COMPOSITION OF THE JUICES, 337 
cells, which, from their content: of resinous matters, 
should imbibe water less readily than other kinds of wood. 
Again, the leaves admit of continual evaporation, and fur- 
nish an outlet to the water. The colored heart-wood ex- 
isting in many trees is impervious to water, as shown by 
the experiments of Boucherie and Hartig. Sap can only 
flow through the white, so-called sap-wood. In early June, 
the new shoots of the vine do not bleed when cut, nor 
does sap flow from the wounds made by breaking them 
off close to the older stem, although a gash in the latter 
bleeds profusely. In the young branches, there are no 
channels that permit the rapid efflux of water. 
Composition of Sap.—The sap in all cases consists 
chiefly of water. This liquid, as it is absorbed, brings in 
from the soil a small proportion of certain saline matters 
—the phosphates, sulphates, nitrates, etc., of the alkalies 
and alkali-earths. It finds in the plant itself its organic 
ingredients. These may be derived from matters stored 
in reserve during a previous year, as in the spring sap of 
trees; or may be newly formed, as in summer growth, 
The sugar of maple-sap, in spring, is undoubtedly pro- 
duced by the transformation of starch which is found 
abundantly in the wood in winter. According to Hartig, 
(Jour. fir Prakt. Ch., 5, p. 217, 1835,) all deciduous trees 
contain starch in their wood and yield a sweet spring sap, 
while evergreens contain little or no starch. Hartig re- 
ports having been able to procure from the root-wood of 
the horse-chestnut in one instance no less than 26 per cent 
of starch. This is deposited in the tissues during sum- 
mer and autumn to be dissolved for the use of the plant 
in developing new foliage. In evergreens and annual 
plants the organic matters of the sap are derived more di- 
rectly from the foliage itself. The leaves absorb carbonic 
acid and unite its carbon to the elements of water, with 
the production of sugar and other carbohydrates. In the 
leaves, also, probably nitrogen from the nitrates and am- 
15 
