572 PHYSIOLOGY. 



generally become converted in the course of time into heart-wood, the 

 solidity of which obstructs the passage of fluids, which then ascend chiefly 

 in the outer, younger layers of wood, which constitute the alburnum or sap- 

 wood. 



This is illustrated by the vegetation of hollow Dicotyledonous trees, in 

 which a sufficient layer of young wood remains within the bark to carry 

 up the absorbed fluids. It is found that the careful removal of the heart- 

 wood of trees does not prevent the supply of liquid to the branches from the 

 roots ; but if the layers of sap-wood are removed, the upper parts of the tree 

 die from desiccation, even when the bark is left uninjured except to such 

 an extent as is sufficient to allow of removing the wood beneath. The 

 removal of a ring of bark does not prevent the ascent of fluid, but, as will 

 be noticed presently, arrests the downward distribution. 



A certain amount of lateral diffusion takes place from the as- 

 cending current, supplying the surrounding tissues with water, 

 and, perhaps, nitrogenous materials ; but this point is not clear. 



Crude Sap. The fluid which is found in the sap-wood of Dico- 

 tyledons is of a watery character, containing dextrine and sugar, but 

 not starch, chlorophyll, or any colouring-matter. It may contain 

 matters dissolved out in its course through the tissues, and thus have 

 a nutritive character from the admixture of matters stored up in the 

 previous season in the wood-cells, &c. It contains also mineral salts 

 absorbed by the roots, in an undecomposed condition, at consi- 

 derable heights in the stem. This fluid is called crude sap. and 

 occurs in especial abundance at the time (spring) when the renewed 

 chemical activity in the developing cellular tissues causes an in- 

 creased absorption of fluids. 



This crude sap flows out freely from incisions into the sap-wood of 

 Dicotyledonous trees in spring, and sometimes spontaneously bursts forth 

 in a kind of overflow, as in what gardeners call " bleeding" of Vines, 

 Birches, &c. 



The crude sap becomes more and more condensed as it ascends in the 

 stem and other organs. In the leaves and other green parts it undergoes 

 a most important transformation, loses by transpiration much of its water, 

 and receives a new element in its composition, of the highest importance 

 to it as material for development, namely carbon, derived from the car- 

 bonic dioxide absorbed by the leaves and decomposed there in sunlight, 

 with the liberation of oxygen. 



Descending Sap. The nature of the progress of the sap 

 from the leaves into the cambium-region of the stem and other 

 parts is at present obscure. Some authors, indeed, totally deny 

 that the elaborated sap descends at all; but this is in contra- 

 diction to all experience and observation. All experiments which 

 have been made favour the opinion that there is a descent of sap 

 elaborated in the leaves, in Dicotyledons at least, in that part of 

 the fibro-vascular bundles coinciding with the cambium-ring of the 



