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THE AMERICAN NATURALIST [Vol. XL 



According to Clark, holes bored at different heights in a birch tree 

 showed that the column of sap was supported almost entirely by 

 the pressure from the root at the base. 



The difference between the bleeding phenomena of the group 

 of trees which bleed in late spring, and then mainly from the root, 

 and the trees of the maple type is, after all, perhaps mainly one of 

 degree only. Although in early spring there is no bleeding from 

 the root but rather suction in the maple, later in April when the 

 leaves are about to appear, root activity is evident here also, though 

 always moderate. Eliminating the peculiar earlier stem pressure 

 the condition in the maple would be normal for the other group. 

 In the case of young maple saplings the root activity begins con- 

 siderably earlier than in large trees, probably due to the shallower 

 root system. This also happens in the roots of young birch sap- 

 lings. In the case of these young maple saplings I am inclined 

 to believe that the flow was perhaps entirely due to root pressure. 



On the other hand, the conversion of starch in the trunk and 

 branch cells of the birch tree may take place at a later date than 

 in the maple and without the accompanying spasmodic changes 

 in the permeability of the membranes. Conversion seems to 

 commence in the root in this case and progress upward, but this 

 point has not been definitely proven. It seems more natural to 

 expect the starch of the trunk in these trees as well as in the maple 

 to be converted before that in the roots owing to the retarding 

 effect of the slowly warming soil. Whether the trunk tissues of 

 birch take part in producing pressure late in the season has not 

 been carefully investigated. From the experiments of Clark it 

 seems rather doubtful. As we have already learned, pressure is 

 not a necessary accompaniment of the escape of sugar from living 

 cells into the vessels. 



If we accept the cell-activity theory for the sap flow in the maple 

 as the most probable, then it seems likely that the difference 

 between the trunk cells of birch and the other trees of this group 

 and those of the maple lies in the inability of the temperature or 

 any other stimulus to cause unequal permeability of tlie pro) 

 nature (i. e., with the proper mechanism) to cause prcssu 

 Therefore the only pressure evident in the birch is the x) cull 

 root pressure. 



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