1902] RISE OF THE TRANSPIRATION STREAM 173 



enasy, 1895 I Dixon, 1896; Darwin, in Darwin, etc., 1896 : 

 336-7 ; Joly*. i'bid, 649) . Regarded purely as a transpiring 

 machine, the live leaf is superior to the dead one chiefly because 

 it does not incapacitate itself by drying. Dixon's suggestion 

 (1896, I) is a good one, that dead leaves transpire less than fresh 

 live ones because the collapse of their cells closes the inter- 

 cellular space. But his other idea, that the fact that flaccid live 

 leaves evaporate only half as much water as dead ones is due 

 to their loss of '* turgescence," seems to me to rest on a confu- 

 sion of the osmotic pressure of the solutes in the cell sap 



J 



as *' turgor — and the state of '*turgescence" caused by the 

 ** turgor" when enough water is av^ailable. It is by virtue of its 



''turgor" that the mesophyll absorbs water from the tracheae; 

 and the ** turgor" is greater, because when the cell shrinks its 

 sap becomes concentrated, in flaccid than in fresh leaves. 



The osmotic pressure in the leaf ceils is much more than is 

 used in lifting water. From very numerous tests by plasmoly- 

 sis I can say that it is in general about equivalent to that of 3.5 

 per. cent KNO3. In some plants it is much higher ; for instance, 

 in grasses and in halophytes. In typical spongy parenchyma 

 the turgor cannot be accurately measured. In the bundle sheath 

 it seems usually to be slightly less than in the palisade tissue, 

 but this may be because the tests of the latter is less accurate. 

 It also seems to be on the whole a little higher in trees than in 

 herbs, but this also ma}- be because the cells are usually smaller 

 m trees, making the turgor appear too high. Dixon (1S96, II) 

 has undertaken to measure the turgor of leaves by ascertaining 

 the gas pressure sufficient to collapse the cells and force water 

 from them into the tracheae. The method is not a reliable one 

 until we know something about the pas^^age of the gases into 

 solution and their diffusion into the cells. We have no infor- 

 mation as to the turgor in the leaves of very high trees, 60 to 

 150"" in height, but to lift the transpiration stream it need not 

 be higher than that of herbs'. 



The mesophyll cells supply themselves with water from the 



