376 HOW CROPS GROW. 



ting a flow of liquid only through perforations of the 

 wood-cells, if these really exist (which Sachs denies). 

 Again, the leaves admit of continual evaporation, and 

 furnish an outlet to the water. The colored heart-wood 

 existing 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 break- 

 ing 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 mat- 

 ters the phosphates, sulphates, nitrates, etc., of potas- 

 sium, calcium, and magnesium. 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, far 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 reports 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 summer 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 directly from the foliage itself. The leaves 

 absorb carbon dioxide and unite its carbon to the ele- 



