264 TRANSFER otf WATER THROUGH THE 



wilt, cutting off the stem a little higher up will cause it to regain 

 in part the power of absorption which it lost upon exposure. 



705. Although osmosis can have very little to do directly 

 with the rapid transfer of water through the stem, branches, and 

 leaves, it plays, as has been seen, a very important part in the 

 introduction of water into the plant, and in supplying the requi- 

 site amount of it to cells which lie, so to speak, away from the 

 main channel of transfer. 



706. Pressure and " bleeding." If, before its leaves unfold, 

 a grape-vine be cut off near the root, or a little higher up on the 

 stem, the cut surfaces will bleed copiously. The part connected 

 with the roots will continue to yield a supply of watery sap for 

 a considerable time. The flow is plainly regulated to a ver}- 

 great degree by the surroundings of the plant, being accelerated 

 by heat and checked by cold. It is not merely passive ; the 

 application of a suitable pressure-gauge shows that the escaping 

 liquid exerts much force. 



One of the early experiments on this subject was made by 

 Hales, 1 who found the pressure in the case of the grape-vine 

 to be equal to thirty-eight inches (105 cm.) of mercury, or more 

 than forty-three feet of water. Other experimenters have 

 reported higher figures; for example, Clark 2 found in Betula 

 lenta a pressure of eighty-five feet of water. 



707. Pitra 8 has shown that a certain amount of pressure is 

 exerted by sap, even in stems which have been severed from 

 the parent plant, the lower extremity being placed in water. 

 In some of his experiments he found that it was not exerted 

 at once, but only after the lapse of a considerable time. He 

 further shows that a considerable pressure is exerted by the 

 sap which flows out of a cut stem the leaves and twigs of which 

 are submerged. 



708. There are considerable individual differences in plants 

 as to the force with which the sap flows from wounds. Wilson 

 found that while one specimen of Ampelopsis quinquefolia gave 



1 Statical Essays, i., 1731, p. 114. 



2 The apparatus for demonstrating the pressure can be easily used. Reduced 

 to its simplest terms, it consists of a mercurial pressure-gauge, which can be 

 securely attached to the wounded part of the plant. To the stump of the 

 plant the gauge must be fastened by means of stout rubber tubing, which has 

 been made to fit tightly around both plant and tube, and then wired firmly 

 to prevent the escape of any liquid. Dahlia variabilis, Vitis vinifera, and 

 Helianthus annuus are good plants for purposes of demonstration. 



8 Pringsheim's Jahrb., xi., 1878, p. 437. 



