426 MR. SPENCER ON CIRCULATION AND 
oceurs a draught upon its contents, the expansion of this air and the entrance of more 
m without, quickly supply the place of the abstracted liquid. 
If, then, returning to the general argument, we conclude that these expanded termi- 
nations of the vascular system in leaves are absorbent organs, we find a further con- 
firmation of the views set forth respecting the alternating movement of the sap along the 
same channels. These spongioles of the leaves, like the spongioles of the roots, being 
applianees 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 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 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 ques- 
tions 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 which, in the spring, while yet there is no considerable evaporation 
to aid it, goes on with a power which capillarity does not explain. The cooperation 
of the same two agencies is assignable for this result also*. The circumstance that 
yessels and ducts here contain sap and there contain air, and at the same place contam 
at different seasons now air and now sap, is a fact calling for explanation. E explana- 
tion is furnished by these mechanical actions which involve the entrance or expulsion 
of air according to the supply of liquid. That vessels and ducts which were originally 
active sap-carriers go completely out of use, and have their function discharged by 
other vessels or ducts, is an anomaly that has to be solved. Again, we are supplied with 
a solution; these deserted vessels and ducts are those which, by the formation of dense 
tissue outside of them, become so circumstanced that they cannot be compressed ?* 
they originally were. A channel has to be found for the downward current of sp 
which, on any other hypothesis than the foregoing, must be a channel separate from 
taken by the upward current; and yet no good evidence of a separate channel has beet 
* It seems probable, however, that osmotic distention is here, especially, the more important of the two factor 
The rising of the sap in spring may indirectly result, like the sprouting of the seed, from the transfor reads t 
edipi During germination, this change of an oxy-hydro-carbon from an insoluble into a soluble form. à 
mapid endosmose, consequently to great distention of the seed, and therefore to a force which thrusts the que 
liquids into the plumule and radicle, and gives them power to displace the soil in their way : it sets up an active = the 
ee when neither evaporation nor the change which light produces can be operative. And similarly, i^ E 
ee - starch stored up in the rcots of a tree passes into the form of sugar, the unusual osmotic absorption j 
ame wit — ye — distention—-a distention which, being resisted by the tough bark of the nt 
result in a powerful upward thrust of the contained liquid. RTE 
mation of starch 
