592 APPENDIX C. 



appreciable resistances; and the network-tissue which they form will, 

 at the same time, be capable of undergoing slightexpansions and con- 

 tractions of its thickness. In this layer occur these organs that are so 

 obviously fitted for absorption. Here we find them in direct commu- 

 nication with its system of collapsible cells. The probability appears 

 to be, that when the current sets into the leaf, it passes through the 

 vessels and their sheaths chiefly into the upper layer of cells (this 

 upper layer having a larger surface of contact with the veins than the 

 lower layer, and being the seat of more active processes) ; and that 

 the juices of the upper layer, enriched by the assimilated matters, 

 pass into the network-parenchyma, which serves as a reservoir from 

 which they are from time to time drawn for the nutrition of the rest 

 of the plant, when the actions determine the downward current. 

 Should it be asked what happens where the absorbents, instead of 

 being inserted in a network-parenchyma, are, as in the leaves of 

 Euphorbia neriifolia, inserted in a solid parenchyma, the reply is, 

 that such a parenchyma, though not furnished with systematically 

 arranged air-chambers, nevertheless contains air in its intercellular 

 spaces ; and that when there occurs a draught upon its contents, 

 the expansion of this air and the entrance of more from without, 

 quickly supply the place of the abstracted liquid. 



If then, returning to the general argument, we conclude that 

 these expanded terminations of the vascular system in leaves are ab- 

 sorbent organs, we find a further confirmation of the views set forth 

 respecting the alternating movement of the sap along the same chan- 

 nels. These spongioles of the leaves, like the spongioles of the roots, 

 being appliances by which liquid is taken up to be carried into the 

 mass of the plant, we are obliged to regard the vessels that end in 

 these spongioles of the leaves as being the channels of the down 

 current whenever it is produced. If the elaborated sap is abstracted 

 from the leaves by these absorbents, then we have no alternative 

 but to suppose that, having entered the vascular system, the elab- 

 orated sap descends through it. And seeing how, by the help of 

 these special terminations, it becomes possible for the same vessels 

 to carry back a quality of sap unlike that which they bring up, we 

 are enabled to understand tolerably well how this rhythmical 

 movement produces a downward transfer of materials for growth. 



The several lines of argument may now be brought together ; 

 and along with them may be woven up such evidences as remain. 

 Let me first point out the variety of questions to which the 

 hypothesis supplies answers. 



It is required to account for the ascent of sap to a height beyond 

 that to which capillary action can raise it. This ascent is accounted 

 for by the propulsive action of transverse strains, joined with that of 

 osmotic distention. A cause has to be assigned for that rise of sap 



