i8 PLANT PHYSIOLOGY 



of the most recent publications on the subject, which brings into the fore- 

 front not theoretical considerations but practical observations (EwART, 1905), 

 such an effect of transpiration is regarded as quite impossible. EWART'S view 

 is shortly as follows : suctional force must be sufficient not only to raise 

 a water column to the top of the highest tree, but must also be able to over- 

 come the resistance which the water meets with in its ascent, a resistance 

 which EWART has measured. He calculated the amount of water which must 

 pass through the stem in a unit of time in order to replace that lost in transpira- 

 tion, and afterwards estimated the pressure required to move this amount 

 at the necessary rate. He found that a water column would be needed which 

 would be 5-7 times as great as the height of the plant. Hence in the case of 

 a tree 150 m. high a pressure of 1,000 m. of water, i. e. 100 atmospheres, would 

 be necessary. If this pressure were induced by the osmotic suction of parenchy- 

 matous cells they would require to possess a cell-sap isosmotic with a 28 per 

 cent, solution of KNO :1 ; EWART holds that that is impossible. 



In this relation it must be noted that the osmotic value of the leaf paren- 

 chyma of lofty trees must be empirically determined by itself ; as high a 

 pressure as that mentioned is improbable but still possible, since in other 

 cells it may reach as high, if not higher values, when the concentration of the 

 external medium is sufficiently great (Lecture XXXIII). EWART has doubtless 

 over-estimated the opposition offered by the wood to the longitudinal carriage 

 of water (compare DIXON, 1906) not only because he took as a basis very high 

 transpiratory values where, in nature, the water reserves would come into 

 play, but also because he possibly did not always take care to keep the cut 

 surface of his experimental twigs free from agents which clog the vessels. 



Every hypothesis as to the significance of the mechanics of water con- 

 duction appears to be founded on an extraordinarily limited groundwork of 

 observation, and to rest on one-sided experimental determinations. 



74, 1. 53, for If the water read If most of the water 



75, 1. 8 P. 76, 1. ig,for A complete discussion . . . Abt. 243) read A detailed 

 account of these views and of the criticisms to which they have given rise 

 (ZIMMERMANN, 1885 ; ScHWENDENER, 1886) need not be presented here ; the 

 fact that water can move as easily upwards as downwards (p. 61) at least 

 does not speak in their favour. 



In what follows we will disregard these special hypotheses and confine 

 ourselves to the question whether the living cells are in any way concerned 

 in the ascent of sap. Attempts have been made to solve the problem by 

 killing longer or shorter stretches of woody stems and seeing whether they 

 still retained their powers of conduction. We have to thank STRASBURGER 

 (1891, 1893) for the most thorough experiments in this direction, experiments 

 which led him to the conclusion that the living cells were quite ineffective. 

 His experiments, however, were not proof against the vigorous criticisms of 

 PFEFFER (1892) and URSPRUNG (1904, 1906). URSPRUNG has conducted a 

 large number of experiments of a similar nature, and arrived at quite different 

 results. He found that, generally speaking, no matter what agent he employed 

 for killing the cells, the conductive capacity for water was injured the more 

 rapidly, the longer the pieces were that were killed. He convinced himself 

 that the deficient conductive capacity was not due to stoppage of the vessels 

 nor to increased evaporation from the dead regions, nor yet to alterations in 

 the JAMIN'S chains. We readily admit that the conclusions he has arrived at 

 from more recent experiments (1906) appear to be much better founded than 

 those he deduced from his previous ones (1904) ; whether, however, his line 

 of argument is above criticism we must leave the future to decide. 



76, 1. 22, for of air read of water 



