No. 474] 



SAP FLOW IN MAPLE 



447 



was simply less responsive and less sensitive, less influenced by 

 the rise in temperature, and much slower to recover after stimula- 

 tion. In this way probably much of the observed individuality 

 existing between different maple trees may be accounted for. 



The quantity of sugar in the sap of any tree would be no in- 

 dication of the difference in permeability, that is of pressure. 

 This sugar content depends upon the total quantity of starch 

 stored in the pith rays and the rapidity of conversion as well as on 

 the permeability. The relatively greater quantity in the sap of 

 the outer wood over that from the inner layers, on the other hand, 

 would probably be proportional to the amount of pressure and 

 flow. 



The flow is not always as great as the pressure would lead one 

 to expect. This may be because in these cases the available 

 supply of water in the wood is less than usual. 



Since each annual cylinder of wood with its system of vessels 

 is in the form of a cone each extending higher on the tree than the 

 preceding, water would probably tend to pass from the shorter 

 layers to the higher ones, thus helping to raise the water in the 

 tree. Water for the inner layers would probably be drawn prim- 

 arily from the soil. Pressure in the twigs and branches is usually 

 much less than in the lower trunk. This may be for three reasons, 

 viz.,— because the temperature would equalize so quickly, because 

 the radial chains of pith-ray cells would be shorter than in the 

 trunk, or because of a less unequal permeability in the cells of the 

 twio-s. When a small branch or twig is cut off, sap flows from 

 })oth surfaces but under no great pressure. I believe that the 

 main portion of the flow in severed branches is due to the com- 

 pressed air caused by the forcing up of sap into this part of the 

 branch from the wood below. 



Sugar probably passes from the wood fibers from the longitu- 

 dinal faces if unequal temperature affects them in a similar manner, 

 and owing to the narrow diameter of the fibers the flow thns caust^i 

 would probably return again to the other side of the fil)ers wit Il- 

 ls the followinij. Why should the excreted sap pass through the 



