CHAPTER XXX 



THE PHENOMENON OF PROPULSION OF SAP 

 AND ITS VARIOUS EFFECTS 



The mechanics of the ascent of sap: (a) Uni-directioned flow — (l>) Initiation of 

 multiple rhythmic excitations — Connection between conduction of excitation 

 and conduction of sap — Rapidity of ascent of sap accounted for by stimulatory 

 action — Positive and negative pressures due to one cause — (i) Positive pressure 

 — (2) Negative pressure — (3) Irregular variations of pressure — Direct con- 

 duction and conduction by relays— Excretion of water — Excretion of nectar — 

 Translocation of organic food-substances — Mechanical response to suctional 

 activity— Effect of warmth — Effect of cold — Explanation of the drooping of 

 leaves during frost — Explanation of response and recovery — Antagonistic 

 actions of internal energy and external stimulus. 



There are various phenomena connected with the transport 

 of water in plants, which are at present considered as entirely 

 distinct. Thus, for example, when a plant is cut above the 

 root, the exuding water exerts a considerable positive pressure 

 on a manometer, this being known as root or exudation 

 pressure. But when manometers are inserted in lateral holes 

 bored in the trunk of a tree, a negative pressure is observed. 

 These facts have led to the inference that there exist, in a 

 transpiring plant, two independent forces, one of suction, and 

 the other of pressure. The negative pressure, again, which 

 is, generally speaking, maximal at the top of a tree, falls to 

 a minimal value near the root. But this fall is characterised 

 by very irregular fluctuations, the negative pressure, in a 

 zone below, being sometimes greater than that at a given 

 distance above. 



I shall now proceed to show, however, that these very 

 various results are not actually due, as supposed, to the 

 operation of distinct forces, but are, on the contrary, so many 

 different effects, under different conditions, of the rhythmic 



