THE FUNCTIONS OF THE WOOD 197 



at least as important a function of the transpiration 

 current as the transference upwards of nutritive mineral 



salts. 



CONCENTRATION OF ELECTROLYTES. 

 With the exception of two observations made in June 

 on Cotoneaster frigida and in October on Populus alba, 

 it appeared that the concentration of electrolytes was 

 greater in the wood of the roots than in that of the stems 

 This would seem to suggest that, while the quantity of 

 dissolved carbohydrates in the transpiration stream may 

 be added to on its upward passage, the amount of dissolved 

 electrolytes is not thus reinforced, but usually is diminished 

 as the stream rises. Doubtless some of the dissolved salts 

 are abstracted and used in various processes of metabolism. 

 The observation on Cotoneaster, however, showed that 

 this does not always hold good, for in it the concentration 

 of electrolytes in the stem was slightly greater than in the 

 roots. Furthermore, in a number of observations made 

 on Acer macrophyllum (see Table LII.) no steady gradient 

 from below upwards is revealed. 



VARIATION IN CONCENTRATION AT DIFFERENT LEVELS 



IN THE CONDUCTING TRACTS. 



The pieces of the stems yielding the sap examined in 

 the foregoing experiments were always taken from the same 

 plant and from the same levels above the ground. These 

 were as follows : 



Acer pseudoplatanus, 8 m. Ilex aquifolium, 1 m. 

 Cotoneaster frigida, 6 m. Populus alba, 12 m. 



Fagus silvatica, 12 m. Salix babylonica, 5 m. 



It seemed desirable to compare the sap from the same 

 level on the different occasions, in case a difference in 



