702 PART IV. —THE PHYSIOLOGY OF PLANTS. 



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

 but 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. In any case the force of the root-pressure is not 

 uniform, but varies ; and the more considerable variations occur 

 in such a way as to constitute a well-marked daily period. The 

 exact periodicity depends partly on the age of the plant, and 

 partly on the conditions under which it has been living: it 

 may be generally described as follows : — The force of the root- 

 pressure is least during the early morning hours ; it then gradually 

 increases, reaching its maximum early in the afternoon, and then 

 it diminishes during the evening and night until the minimum 

 is attained early the following morning. Thus there is a period 

 of about twelve hours between the occurrence of the minima and 

 the maxima, and there can be no doubt that this periodicity has 

 been induced by the periodic changes in the external conditions 

 accompanying the alternation of day and night. 



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 parenchymatous 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. 701), 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 de- 

 tected at any time in a great many plants, and that it can never 

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

 when, on the contrary, there is negative pressure (p. 699) 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. 



It has been suggested that the current is maintained by a 

 repetition of the root-pressure-action at various levels in the stem, 



