442 



THE AMERICAN NATURALIST 



[Vol. XL 



of the cells first, acts as a stimulus and causes the ray cells to be- 

 come more permeable to sugar at one end, which is the same end 

 in every case. Since the greater pressure and sugar content is in 

 the outer wood, it seems probable that the outer ends of the ray 

 cells rather than the inner become more permeable, and the flow 

 would be therefore from the heart wood toward the cortex. In an 

 untapped tree of course there would be scarcely any flow, simply 

 statical pressure, but sugar would pass into the vessels just the 

 same. But the mechanism cannot be quite as simple as it seems 



at first. It will not do to assume simply that the membrane at 

 one end of the cell becomes more permeable to the solute in both 

 directions than does that at the other end. Let A and B in Fig. 

 3 each represent three pith-ray cells, and the intervening shaded 

 portions the cell walls between. Sujjpose that in A, at the x 

 membranes of each cell, there is a tendency Mo 15 gm. osmotic 

 pressure, while at the // iiicinbraiics tliere is a tendencv 



static pressure must really 1 



o only Jj 

 5 hydro- m 



