286 PHYSIOLOGICAL DEVELOPMENT. 



from that direction most must return during the dilation; 

 seeing that the force which more efficiently resisted the 

 thrusting back of the sap is the same force which urges it 

 into the expanded vessels again, when they are relieved from 

 pressure. At the next bend of the part a further portion of 

 sap will be squeezed out, and a further portion thrust for 

 wards along the vessels. This rude pumping process thus 

 serves for propelling the sap to heights which it could not 

 reach by capillary action, at the same time that it incident 

 ally serves to feed the parts in which it takes place. It 

 strengthens them, too, just in proportion to the stress to be 

 borne; since the more severe and the more repeated the 

 strains, the greater must be the exudation of sap from the 

 vessels or ducts into the surrounding tissue, and the greater 

 the thickening of this tissue by secondary deposits. By 



this same action the movement of the sap is determined 

 either upwards or downwards, according to the conditions. 

 While the leaves are active and evaporation is going on from 

 them, these oscillations of the branches and petioles urge 

 forward the sap into them; because so long as the vessels of 

 the leaves are being emptied, the sap in the compressed 

 vessels of the oscillating parts will meet with less resistance 

 in the direction of the leaves than in the opposite direction. 

 But when evaporation ceases at night, this will no longer be 

 the case. The sap drawn to the oscillating parts, to supply 

 the place of the exuded sap, must come from the directions 

 of least resistance. A slight breeze will bring it back from 

 the leaves into the gently-swaying twigs, a stronger breeze 

 into the bending branches, a gale into the strained stem and 

 roots roots in which longitudinal tension produces, in an 

 other way, the same effects that transverse tension does in 

 the branches. 



Two possible misinterpretations must be guarded against. 

 It is not to be supposed that this force-pump action causes 

 movement of the sap towards one point rather than another : 

 it is simply an aid to its movement. From the stock of sap 



