i9o8] CURRENT LITERATURE 545 



is a scheme for corn breeding worked out in very clear detail by C. G. Williams, 

 by which it is hoped to secure the greatest possible improvement without encoun- 

 tering the injurious effects of inbreeding. In the rapid development of all breeding 

 problems, due to the universal interest which is being taken both in scientific and 

 economic aspects of the subject, the annual reports of the American Breeders' Asso- 

 ciation must continue to be a mostValuable source of information as to the present 

 state of knowledge, and no one who is interested in these problems from either 

 point of view can afford to be without the annual volumes— George H. Shull. 



NOTES FOR STUDENTS 



Ascent of water.— E wart becomes a stronger and stronger opponent of tlie 

 DixoN-JoLY theory of the ascent of sap, and in his last paper" adduces some 

 powerful arguments, backed by observations, against it and in favor of the thec^ry 

 of vital maintenance of suitable conditions for conduction and some sort of pump- 

 ing action. The latter may be by surface tension, but while theoretical ways 

 abound in which this might be applied, no practical proof of the existence of any 

 such action in wood is forthcoming. Ewari's experiments all tend to show he 

 thinks, that the continuous ascent of water is possible only in living wood, and that 

 the power of conduction is rapidly lost at death. Experiments on the suction and 

 exudation of trees at different levels showed no continuous water columns or 

 high internal tensions in the tracheae during active transpiration, and this fact, 

 coupled with the high total resistance to flow, indicates that this resistance i. o^er- 

 come locally from point to point, and not by enormous tension from above, « hic 

 leads to blocking by gas bubbles, nor by pressure from below, which leads o 

 great loss by lateral exudation from the vessels. 



Ew.^T makes several corrections of his own and others' previous «bser\ ations^ 

 He finds no vessels open the whole length of the tree, as StR-^sbl-RGER though lo 

 be the case in oak. Wistaria furnished him the longest-5 • 6^- In the oa > 

 are seldom over i™. Nor does Ewart sustain StR-^sburger's results on me 

 conduction of water through dead wood, for if these experiments ^re sound, u 



action is out of the question. His own earlier observations on the «^"^« ^^ P^^^^J^J^ 

 in leaves at different levels are also disowned, for he now finds as grea ' "■ 



in leaves at the same level. Incidentally he looked up the endence as to tn 

 height of the tallest trees in Australia, and concludes that none W^^bl) ^^ce 

 300 feet, instead of being 472 as reported. He thinks that the height of some 

 our American big trees may likewise shrink on investigation. ^^^ ^^^^ 



In the course of the discussion of exudation he makes the very goo F'^ ^^^^ 

 fully turgid cells, for example, those of the root cortex, may act simp >^^^ ^^^^^_^ 

 brane between the water in the vessels and the outside water ^^^''""'^^^^ ^.j ^^ 

 pressure, however high, having no influence on the transfer of jva er 

 vessel so long as they remain fully turgid. This also e.xplains how ceiis 



« Ewart, A. J., The ascent of water in trees (second paper). Phil, 

 ooc. London B 199:341-392. igoS. 



