THE FORMATION OF WOOD IN PLANTS. 425 
The cotyledons of the young bean, experimented upon as described at page 410, fur- 
nished other examples of such structures, exactly in the places where, if they are absor- 
bents, we might expect to find them. Amid the branchings and inosculations of the 
vascular layer running through the mass of nutriment deposited in each cotyledon, there 
are conspicuous free terminations that are club-shaped, and prove to be composed, like 
those in leaves, of irregularly formed and clustered fibrous cells; and some of them, 
diverging from the plane of the vascular layer, dip down into the mass of starch and 
albumen which the young plant has to utilize, and which these structures can have no 
other function but to take up. 
Besides being so well fitted for absorption, and besides being similar to organs which 
we cannot doubt are absorbents, these vascular terminations in leaves afford us yet 
another evidence of their functions. "They are seated in a tissue so arranged as specially 
io facilitate the abstraction of liquid. The centripetal movement of the sap must be 
set up by a force that is comparatively feeble, since, the parietes of the ducts being 
porous, air will enter if the tension on the contained columns becomes considerable. 
Hence it is needful that the exit of sap from the leaves should meet with very little 
resistance. Now were it not for an adjustment presently to be described, it would 
meet with great resistance, notwithstanding the peculiar fitness of these organs to take 
itin. Liquid cannot be drawn out of any closed cavity without producing a collapse of 
the cavity’s sides; and if its sides are not readily collapsible, there must be a cor- 
responding resistance to the abstraction of liquid from it. Clearly the like must happen 
if the liquid is to be drawn out of a tissue which cannot either diminish in bulk bodily 
or allow its components individually to diminish in bulk. In an ordinary leaf, the 
upper layer of parenchyma, formed as it is of closely packed cells that are without inter- 
spaces, and are everywhere held fast within their framework of veins, can neither con 
tract easily as a mass, nor allow its separate cells to do so. Quite otherwise is it with 
the network parenchyma below. The long cells of this, united merely by their ends and 
having their flexible sides surrounded by air, may severally have their contents con- 
siderably increased and decreased without offering appreciable resistances ; and the net- 
work tissue which they form will, at the same time, be capable of undergoing slight 
expansions and contractions of its thickness. In this layer occur these organs that 
are so obviously fitted for absorption. Here we find them in direct communication 
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 — 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 nutri- 
tion 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 nerüfolia, inserted in à solid parenchyma, 
the reply is, that such a parenchyma, though not furnished with systematically arranged 
ür-chambers, nevertheless contains air in its intercellular spaces; and that when there 
