204 



BOTANY 



PART I 



conducting system 



All the wood of a thickened stem does not serve 

 this purpose ; water conduction is limited to the more recently 

 developed annual rings. When a heart-wood is formed this takes 

 absolutely no part in the process. 



There is still uncertainty as to the forces which give rise to the 

 transpiration stream. It is natural to think of a pressure acting from 



below, or a suction from 

 above, and to regard the 

 former as due to root-pres- 

 sure, the latter to the pro- 

 cess of transpiration. There 

 are, however, a number of 

 reasons against ascribing 

 the movement of the water 

 to root- pressure, and 

 whether the suction force 

 exerted by transpiration is 

 sufficient to continuously 

 raise water to the summit 

 of a high tree appears 

 doubtful. No generally 

 accepted solution of the 

 much-discussed problem of 

 the ascent of water has 

 yet been attained. 



The following points have to 

 be considered as regards the 

 root-pressure. lu many plants 

 the root-pressure actually ob- 

 served is very slight or absent. 

 Even in plants with a powerful 

 root-pressure the amount of 

 water thus supplied in a given 

 time is considerably less than 

 that lost in transpiration. With 

 somewhat more active tran- 

 spiration, therefore, the root- 

 pressure is not manifested in 

 the way described above. When an actively transpiring plant is cut across above 

 the root, no water is at first forced from the stock ; but, on the other hand, if water 

 is supplied to it the cut surface absorbs it greedily. Only after it is fully saturated 

 does the forcing-out of water commence. In nature root-pressure thus only comes 

 into play when transpiration is greatly lessened, for instance at night when the 

 air is damp and cool. The most favourable conditions for this phenomenon occur 

 in spring when, on the one hand, the wood is richest in water, and, on the other, 

 the transpiring foliage is not fully developed. On wounding the xylem the sap 

 then oozes in drops out of the vessels and tracheides. 



That transpiration induces a suction from cell to cell has been pointed out 



Fin. 190. — Course of the vascular bundles (venation) in a 

 \eaf of Crataegus. (From a photogi'aph ; natural size.) 



