561 



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

 at the same time, be capable of undergoing slight expansions 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 communica- 

 tion 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 mto 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, em-iched 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 parenclmna, are, as in the leaves of 

 Euphorbia neriifolia., inserted in a sohd parenchyma, the reply is, 

 that such a parenchyma, though not furnished with systematically 

 arranged air-chambers, nevertheless contains air in its intercellular 

 S])ace3 ; 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 absorbent 

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

 mg the alternating movement of the sap along the same channels. 

 These spongioles of the leaves, like the spongioles of the roots, being 

 apphances by which hquid is taken up to be carried into the mass of 

 the plant, we are obhged 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 elaborated 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 unhke 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 fines of argument may now be brought together ; and 

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

 me first pomt out the variety of questions to which the hypothesis 

 suppfies answers. 



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

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

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

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



