THE CONDUCTION OF WATER. I 6i 



Weinrowsky. 1899. Fiinfstiick's Beitr. 3. 

 Westermaier. 1884. Sitzungsber. Berlin. Akad., p. mo. 

 WiELER. 1893. Cohn's Beitr. z. Biol. 6, i. 

 Wilson. 1881. Unters. aus d. bot. Inst. Tiibingen, i, 8. 



LECTURE VI 



THE CONDUCTION OF WATER. II 



Having seen how water enters the vessels, we have now to inquire as to 

 the forces which bring about its ascent to the tops of lofty trees. In order to 

 obtain some idea of the amount and direction of these forces it will be necessary 

 for us first of all to get a clear conception as to the course, the quantity, and 

 the rapidity of carriage of the water, and of the height to which it is carried. 

 As to the direction followed by the water, there is no question, at least in all 

 ordinary cases, viz. from below upwards, from the absorbing root to the tran- 

 spiring leaf. It is important to note, however, that a current may be formed 

 in the reverse direction, there being no special appliances in the interior of 

 the vessels for guiding the flow in one direction only. Indeed the older physio- 

 logists who studied the subject held strongly that water could move as easily 

 from the apices of the branches down to the root as in the normal direction. 

 More recently, Th. Hartig (1861) showed that solutions of certain substances 

 could travel through the wood of felled trees and of isolated branches in the 

 reverse direction, if the upper parts were submerged instead of the lower. 

 Much the most convincing, however, was the experiment performed by Stras- 

 BURGER on the beech (1891, p. 938). He employed a stem which had fused 

 high up with a neighbouring stem, the latter being abundantly provided with 

 leafy branches right down to the ground. He severed this stem at its lower 

 end so that both it and its branches were entirely dependent on the water which 

 the other stem had absorbed, although in order to reach the lowest branches 

 the water had to flow from above downwards. These branches, however, 

 remained quite fresh for several years (Strasburger, 1893). The experiment 

 showed, in addition, that the amount of water which flowed in the reverse 

 direction to the normal was amply sufficient to maintain turgidity in the leaves 

 of the lowest branches. 



The amount of water transpired gives us some indication as to the quantity 

 which normally travels up a tree trunk. A glance at the flaccid leaves of 

 plants, as they appear in the evening of a hot summer day, shows clearly that 

 more water is evaporated from the leaves than is carried up to them. But since 

 the leaves by the following morning have again become rigid, it follows that 

 they must have recouped themselves during the night for what they have lost by 

 day. Generally speaking, it may be said that the amount of water transpired 

 during the twenty- four hours is roughly equivalent to what is lifted in the 

 same time. These amounts might be found identical were it possible to prove 

 that the amount of water contained in the wood showed no variations, but that 

 is not very probable. We owe our knowledge of the amount of water contained 

 in tree trunks at different seasons of the year to the laborious researches of 

 R. Hartig (1882). Unfortunately each determination necessitated the felling 

 of the whole tree, so that it was impossible to estimate how much of the result 

 was due to individual variations and how much to the season of the year. 

 When one remembers that research has shown that, in the case of the Scotch 

 pine, 50 per cent, of the entire volume of the wood, according to one estimate, 

 and only 25 per cent, according to another, was water, we are compelled to 

 assume the existence of variations in water capacity in each stem, and to look 



