THE FUNCTIONS OF THE WOOD 205 



when they were maintained for several days in almost 

 total darkness, being still attached to the tree the other 

 leaves of which were assimilating actively. When the 

 weather conditions interfered with the formation of sugars 

 by the exposed leaves, the concentration of these substances 

 in the covered ones decreased very markedly. 



Under normal summer conditions in, for example, Syringa 

 there is a decrease in concentration from the leaf cells, 

 through the bast, to the cells of the medullary rays. Trans- 

 formation of sugars into starch takes place in the latter, 

 hence the low pressure is maintained in this region. 



It was pointed out, when dealing with the permeability 

 of the plasmatic membrane, that sugars are frequently 

 found to pass through the protoplasm at a quite appreciable 

 rate. Since the wood parenchyma cells are in contact with 

 a very dilute solution of mineral salts, it is quite possible 

 that the " secretion " of sugars into the stream may in 

 reality be accounted for by simple diffusion, through the 

 plasmatic membrane. Furthermore, as Osterhout has 

 shown, the permeability of the latter may undergo repeated 

 increases or decreases within certain limits quite unaccom- 

 panied by any injurious effects. From such considera- 

 tions as these one is led to the view that this " secretion " 

 is not necessarily an instance of the intervention of the 

 protoplasm as an active agent, but that it is, perhaps 

 preferably, to be thought of as due to diffusion. Though 

 the writer advances this view, he does not wish in any way 

 to throw doubt on the fact that living cells can secrete 

 concentrated solutions when supplied with dilute ones, 

 and in other ways act in a manner that, when looked at 

 with the comparatively superficial knowledge of the present 

 day, appears to be in opposition to physico-chemical laws. 



