160 The Plant World. 



siderations (p. 218) that Fitting formulated the experimental 

 project which he has so he successfully carried out. 



The supposition that high pressures in the leaves indi- 

 cate correspondingly great powers of absorption from dry soils 

 it the form in w'hich it is advanced by our author, depends iipon 

 the assumption that these pressures are accompanied by similar- 

 ly hiqh pressures in the root cells (which has been rendered 

 highlv probable by Stange — loc. cil. — -,h\\i \\hich needs more 

 thorough study) and also upon the assumption that the entrance 

 of ivatcr into the root is a function of the osmotic-capillary or 

 osmotic-adsorption relation described in the last paragraph. 

 This last assumption is by no means an established fact; we 

 know as vet practically nothing of the energy relations of w-ater 

 a' sorption from non-saturated soils, and the work of Dixon on 

 tlie tensile strength of minute water columns '^ renders it 

 apparentlv possible that "osmotic pressure does not indeed 

 play the important part in water absorption which has hitherto 

 been assigned to it. ' ' * This one of the two alternatives i)re- 

 sented bv mv experiments with soil-osmometers meets with 

 no approval from Fitting (p. 219). I must, however, remain 

 undecided in regard to this whole question of the physics of 

 root absorption till more quantitative information is at hand 

 (the question will be referred to again below), but agree thor- 

 ouglilv with Fitting that "this question can be answered by 

 the research-methods of physical chemistry" (p. 219), and that 

 the osmometer method described in Publication 50 "might have 

 been replaced by a better" (p. 220). But, after several years if 

 experimental contact with this problem of the resistance offered 

 b\ the soil to root absorption, 1 am unable to agree wath 

 him that a satisfactory method may be attained "without 

 difficulty '("wohl unschwer, " p. 220). This is perhaps the 

 most important problem that confronts the plant physicist 

 todav, and is well worthy of careful study. 



One rather fundamental aspect of the relation between 

 soil and roots, at least so far as my own studies in this connection 

 have led me, is one that seems completely to have es- 



*Sum;naii'y prpsenlcd in this p;ipcr, "Transpiration and the Ascent of Sap," Prog. 

 Rei. Bot. 3: 1-66. 1900. 

 tPub'iitation 50 of the Carnegie Institution, 1906, p. 21. 



