388 SUMMARY OF CURRENT RESEARCHES RELATING TO 



to flow towards the cut surface. The insoluble salt collects at the 

 open mouths of the vessels, often passing into the capillary tubes after 

 a tenaporary arrest, and the same phenomenon is repeated several times 

 as the minute plugs are formed and then sucked in. 



With low powers of the Microscope it is possible to use a second 

 slip instead of the thin cover, and then the simple apparatus can be 

 held more firmly in its place. In any case it is possible to measure 

 the rapidity of the current by means of a micrometric eye-piece ; and 

 several such measui'ements are given. 



When the stem is quickly stripped of its leaves, the current is 

 stopped at once. But when, on the other baud, a leaf or a part of the 

 stem is pinched, there is immediately a backward flow of water. 



It is well known that two conflicting views have been held by 

 physiologists as to the channel by which the ujDward movement of 

 water in wood takes place. Some think that the transfer is solely by 

 imbibition, and that no free water is carried from cavity to cavity of 

 the wood-element, or rather, that no free water exists in the cavities. 

 Others have held that free water is carried from one wood-element to 

 another, and that the walls themselves play only a subordinate role. 

 To these opposed views may be added a third, which appears to be a 

 compromise ; namely, that water in a free state actually exists as a 

 thin lining on the cell-wall. The chief advocate of the latter view 

 has, however, abandoned it in favour of the imbibition theory. A 

 recent publication by Elfving* details the results of experiments 

 which considerably strengthen the "cavity" theory. Now just at 

 this point come observations of Vesque, in a continuation of the paper 

 regarding the method of direct demonstration, which go far towards 

 showing that here, as was long ago suspected, the truth is to be found 

 between the extremes. These experiments,' which need to be care- 

 fully repeated, indicate that under certain circumstances the transfer 

 of water takes place by means of the cavities themselves, but that in 

 all cases they may serve the part of reservoirs. 



Moreover, the calibre and length of the vessels regulate the rate 

 of transpiration ; resistance to the movement of the water following 

 the law of Poiseuille, so that the resistance is inversely proportional 

 to the fourth power of the diameter, and directly proportional to their 

 length. We give in full the close of Vesque's paper. 



" It is evident that p having reached its maximum, that is to say 



the suction resulting from transpiration not being able to increase 



without changing our conditions, because the air dissolved in the 



water becomes disengaged, the quantity of water which arrives at the 



organs of transpiration across a vessel filled with water is expressed 



A (Z* 

 by — y-. From this we can see why climbing plants have such 



large vessels ; in fact, the increase in diameter can alone compensate 

 that of the length. And, further, the qiiantity of water which can 

 pass through a vessel in a given time bears a certain relation, varying 

 for each species, with the water which it contains. This, which I have 



* See infra, p. 389. 



