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 efliux 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 2>hosphates, 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 rnay 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. fiXr 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 uge 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 ar!.d ui^ite 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 3,m- 

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