KATK OF ASCKNT IN THE STEM. 263 



sul-e in contiguous cells. ... In parencliyuiatous tissues filled with 

 sap the movement of water caused by evapoi-ation is a function 

 of the elasticitj- of the cell-walls and of atmosi)henc pressure." 



Herbert Spencer has shown that when a cut stem is quickly 

 bent backwards and forwards tiiei-e is a marked increase iu tlie 

 rapidity- with which colored fluids ascend through it. " To 

 ascertain the amount of this propulsive action, I took from the 

 same tree, a Laurel, two equal shoots, and, placing tbem in the 

 same dye, subjected them to conditions that were alike in all 

 respects save that of motion : while one remained at rest, the 

 othrr was bent backwards and forwards, now by switching and 

 now by straining with the fingers. After the lapse of an hour 

 I found that the dye had ascended the oscillating shoot three 

 times as far as it had ascended the stationary shoot, this re- 

 sult being an average from several trials. Similar trials brought 

 out similar effects in other structures."^ 



704. Effect upon transfer of exposing a cut surface to the air. 

 One of the most interesting characteristics of the wood^' tissues 

 in relation to the transfer of water is the immediate change 

 which the cut surface of a stem undergoes upon exposure to air, 

 unfitting it for its full conductive woi-k. De Vries -^ has shown 

 that when a shoot of a vigorous plant, for instance a Hehanthus, 

 is bent down under water, care being taken not to break it even 

 in the slightest degree, a clean sharp cut will give a surface 

 which will retain the power of absorbing water for a long time ; 

 while a similar shoot cut in the open air, even if the end is in- 

 stantly plunged under water, will wither much sooner than the 

 first. Shoots cut in the manner first described remain turgescent 

 for several daj-s. If a cut shoot placed in water has begun to 



against the entrance and exit of water, and one of the two surfaces remaining 

 uncovered was placed in water, the other exposed to air, when the transfer of 

 water through the wood was found to be more rapid in a longitudinal tlian in 

 a transverse, and in a radial than in a tangential direction. 



Another method of experimenting was also employed by him : five sides of 

 a cube of wood were surrounded by separated portions of dry calcic chloride, 

 and the remaining side was placed in contact with water ; the difference in 

 rate of transfer ascertained by comparing the weights of the portions of calcic 

 chloride after a fixed time was found to be essentially that given by the other 

 method. 



Experiments by Sachs (Arbeiten des botan. lustituts in Wiirzhurg, 1879, 

 p. 298), in whichwater was forced in different directions through the wood of 

 coniferous stems, showed, however, that under pressure water passes through 

 •wood more readily in a tangential than in a radial direction. 



^ Transactions of Linnfean Society, xxv., 1866, p. 405. 



2 Arbeiten des botan. Inst, in Wtirzburg, i., 1874, p. 292. 



