688 
MOLECULAR FORCES IN THE PLANT. 
exerted also against the inside of the vessels which receive the water from the turgid 
parenchymatous cells. But the water which enters them has also to overcome the 
resistance to filtration exercised by the cell-walls. The endosmose of the cortical cells 
of the root must overcome these resistances. Although we do not know the magnitude 
of the endosmotic force, yet we have ground for supposing that it is much greater than 
that given by Dutrochet's experiments on animal membranes; and this explanation would 
therefore be very probable. But a difficulty occurs in answering the question why the 
turgescent cortical cells of the root expel their yvater only inwards into the woody tissues 
and not also through their outer walls. We may however here be helped by the suppo- 
sition that the micellar structure of the cell-walls is different on the outer and inner 
sides of the cells, and that those facing the exterior of the root are best adapted to allow 
endosmose, while those facing the interior of the root are best adapted for permitting 
filtration under high endosmotic pressure. It must however be observed that this 
supposition is at present only a hypothesis for the purpose of explaining to a certain 
extent the processes which take place in the root. The exudation of drops of water 
from the upper cell of the Fungus Pilobolus crystall'mus, from the root-hairs of a Mar- 
chantia grown in damp air, &c., shows moreover that cells distended by endosmotic 
tension can in fact exude water at certain spots. It is difficult to give any other ex- 
planation of the exudation of nectar in flowers ; the excreting cells must evidently absorb 
the water or the sap with great force on one side, and then exude it on the other side. 
That in this case pressure from the root does not directly cooperate is shown by the 
fact that this exudation of nectar, which is often very copious, as in the flowers of Fritil- 
laria imperialis and in the pitchers of Nepenthes^, takes place even when cut flowers or 
pitchers are simply placed in water. In this respect these exudations of fluid differ from 
the exudation of drops on the leaves of many plants, which only takes place when they are 
still in connection with the root, and which is clearly caused by the forcing power of the 
root (as in Aroideae, &c.). It also happens however sometimes that drops of water are 
exuded from cut surfaces of the tissue, while another cut surface of the organ absorbs 
water. This I found, for instance, to be the case with pieces of the young stems of 
diff'erent Grasses, cut off" from 6 to lo cm. in length, which, were placed with the lower 
end in damp soil ; the free upper end then repeatedly and continuously exuded drops of 
water in darkness and in an atmosphere saturated with moisture. Here the paren- 
chymatous cells of the lower cut surface clearly acted as the cortical cells of the root, 
absorbed by endosmotic action, and probably pressed the water thus absorbed into the 
vessels, from which it then escaped to the upper cut surface. 
(y) The combined action of transpiration, conduction, and absorption of ivater by the 
roots takes place under ordinary and favourable conditions in such a manner that nearly 
as much water is absorbed through the roots and conducted upwards through the 
wood as is transpired from the leaves. As long as this equilibrium lasts, the plant is 
turgid and tense in all its parts ; and conversely, it may be concluded from the unaltered 
turgidity and tenseness of the leaves and internodes that the conduction of water is 
sufficient to compensate the transpiration by the leaves. Hence, under these conditions, 
the quantity of water transpired may be taken as the measure of the absorption of the 
root (or of a cut surface), and conversely the absorption observed as the measure of the 
^ Compare Wunschmann's dissertation, 'Ueber die Gattung Nepenthes' (Berlin, 1872), where my 
Handbook only, and not the above taken from the third edition of this work, is quoted. It is 
questionable if there is any ground for distinguishing between 'excretions' and the sap which escapes 
from a root-stock in so far as the mechanism of the excretion is concerned, as the older botanists and 
Wunschmann do. The facts above mentioned render it improbable. They tend to show, on the 
contrary, that in other vegetable organs, as well as in roots, hydrostatic pressures may be set up 
which tend to force the fluids out of the tissues. It is a matter of merely secondary importance that 
'excretions,' such as nectar and the fluid contained in the pitchers of Nepenthes, are of higher concen- 
tration than the sap which escapes from a root-stock. 
