44] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 183 



xylem, their protoplasm undergoes a molecular change, in conse- 

 quence of which it becomes permeable and ceases to offer resistance 

 to the escape of the cell-sap ; consequently, under the elastic con- 

 traction of the distended cell- walls, a portion of the cell-sap is forced 

 out of the cell into the vascular tissue. From this point of view, 

 the root-pressure of a plant is simply the expression of the force of 

 the elastic contraction of the cell-walls of the parenchymatous cells 

 abutting on the xylem-bundles in the root. 



With regard to the external conditions which affect the root- 

 pressure, the most important is the temperature of the soil ; a 

 rise of temperature up to the optimum increases the root-pressure, 

 bat any further rise causes it to diminish, and if the soil be 

 heated so as to kill the roots, the root-pressure altogether dis- 

 appears. 



The liquid forced into the tracheal tissue is by no means pure 

 water; it holds various substances in solution, such as mineral 

 salts absorbed from the soil ; in the spring it is relatively rich 

 in organic substances, such as proteids, sugar, acids, colouring- 

 matters, etc., derived from the reserves stored in the parenchy- 

 matous cells of the root, which are being conveyed to the opening 

 buds. 



The Transpiration-Current. The mechanism by which, after 

 the liquid has been forced into the xylem of the root, a sufficient 

 current is maintained through the stem of a lofty tree to supply 

 the actively transpiring leaves, is still one of the incompletely 

 solved problems of physiology. 



It might be assumed that the transpiration-current is main- 

 tained simply by the root-pressure. There is no doubt that, in 

 low-growing plants (see p. 180), the root-pressure is sufficient to 

 force liquid to all parts of the plant ; and this is probably true 

 also of lofty trees. The objection is that no root-pressure can be 

 detected in any plant at the time when transpiration is active, 

 when, on the contrary, there is negative pressure (p. 180) in the 

 vessels. Moreover, a transpiration-current is maintained for a 

 time by entire plants whose roots have been killed by heat, as also 

 by cut-off shoots. 



The present position of the question as to the mechanism of the 

 transpiration-current in lofty trees, may be stated as follows. In 

 the spring the wood is full of water forced into it by root-pressure. 

 When the leaves unfold and begin to transpire, water is gradually 

 withdrawn from the conducting tracheal tissue, and the tissue is 



