100 ORGANS AND FUNCTIONS OF PLANTS. 



and return white from the edges of the leaf. By simi- 

 lar experiments on the apple and horse-chestnut trees, 

 T. A. Knight traced the pulp from the leaf into the 

 leaf-stalk, and also into the inner bark, through which 

 it probably descends to the root. 



The descent of the pulp through the innermost layers 

 of the bark is shown by the experiments of Du Ham el 

 and De Sarabat, who cut a ring of bark from a branch, 

 and found that by thus stopping the descent of the 

 pulp, the upper part extended and healed, while the 

 lower remained stationary. 



The moving force which causes the pulp to descend, 

 is no better understood than the power which makes 

 the sap ascend. 



The Pulp Cells. 



As the superfluous nourishment of animals is con- 

 tained in cellular tissue in the form of fat, in a similar 

 manner, it would appear, is the superfluous or over- 

 plus pulp stored up in the cells of plants, or, as Link 

 terms them, cellular reservoirs. 



These reservoirs are not uniformly met with, but 

 occur irregularly in the bark, and sometimes in the 

 pith. Malpighi describes them in the bark of the oak 

 and poplar, as containing concrete pulp ; and De Can- 

 dolle supposes the pulp to nestle throughout a plant, 

 in cells, sacs, or reservoirs of various sizes; thus con- 

 founding, according to Ellis, the superfluous and extra- 

 vasated pulp with what is indispensable for nutrition. 



M. Dutrochet describes cells of a peculiar kind, of 



