THE METHOD OF MOVEMENT 173 



the soil solution penetrates along the walls of the root cortex 

 cells. Neither of these types of cells have the peculiar thick 

 walls of the phloem region. As has been stated in Sec. 22, 

 the flow within walls of the root cortex is based chiefly on 

 the meager evidence of the staining of the walls. The 

 suggested flow within the walls into the meristem is also 

 largely assumption with little or no supporting evidence. 

 Priestley's chief reasons for suggesting the walls as the 

 channel for transport are that living membranes show high 

 impermeability and that a rapidly developing cell would 

 be unlikely to allow necessary foods to pass through to 

 other cells. If solutes diffuse from cell to cell through 

 plasmodesma and not through surface membranes, the first 

 reason offered loses its force as does perhaps the second 

 reason also. Furthermore the mechanism suggested by 

 Priestley to account for forcing the sap into the meristem 

 is not based on very positive evidence. 



The mechanism, as I understand it, is that an isolated, 

 maturing xylem element, or group of elements, as it becomes 

 permeable, releases a rather concentrated solution which 

 draws water osmotically across the membranes of enclosing 

 cells. This sap, rich in foods, is thus forced within the 

 walls to the meristem. This explanation of the method 

 of the nutrition of the growing point is based on three 

 assumptions each of which appears to me not only unsup- 

 ported by direct evidence but even highly improbable. In 

 the first place it is assumed that the xylem elements, in 

 maturing and losing their protoplasmic membranes, release 

 a relatively concentrated solution. To me it seems more 

 likely that by the time the cell approaches death or loss 

 of its membrane it has used up practically all of its available 

 organic material in respiration and in forming the cell wall ; 

 thus little or none would be left either for developing the 

 pressure necessary to force the solution along the walls or 

 for nourishing the meristem. In the second place it is 

 assumed that this solution would be forced toward the 

 meristem, but it is not clear as to what would prevent its 

 leakage in all directions away from as well as toward the 



