Hoagland —92— Plant Nutrition 



from one point of view a process equivalent to secre- 

 tion. According to this proposal it would not be 

 essential that all the salt absorbed and transported 

 to the xylem vessels first be accumulated in cell 

 vacuoles. It is hoped to gain further information 

 on this point by studies with isotopes. 



Path of Upward Movement of Salt : — It becomes 

 now desirable to consider further the question of 

 inorganic solute movement and its accumulation in 

 the upper parts of the plant before embarking on 

 additional discussion of the interrelations of root and 

 shoot. 



As I have already said, several investigators have 

 suggested the possibility that the upward movement 

 of salt occurs primarily in living cells of the phloem, 

 especially in the sieve tube system. Much of the orig- 

 inal basis for this idea was found in experiments 

 conducted over long time periods with woody plants, 

 by the method of removing rings of bark from the 

 branches to be subjected to examination. A difficulty 

 with experiments of this kind is that secondary effects 

 of ringing can occur, which may, for example, impair 

 the nutrition of the roots, or lead to injury of the 

 conducting system of the xylem. Transpiration may 

 also be affected by accumulation of carbohydrate above 

 a girdle. Despite these complications, Clements and 

 Engard (1938) found in tests on one woody species, 

 with a diffuse porous wood, that practically as much 

 salt moved upward through a girdled branch as 

 through an ungirdled one and that large amounts of 

 salt moved upward past a girdle in all species studied 

 although in most of them girdling decreased mark- 

 edly the upward movement of salt. Clements, and 

 also F. C. and A. G. Steward,* from another point 

 of view, have emphasized the problem presented in 



*Private communications. 



