MOVEMENT OF WATER IN PLANTS. 
679 
difficult in the extreme ^ The cuticularised outer wall of the epidermis of young 
leaves and internodes is less efficacious in this respect ; if it is very thin as in many 
quickly-growing leaves, especially those of water-plants, or altogether imperceptible as 
in roots, these parts dry up very quickly in ordinary air. In contradistinction to this 
the evaporation is very small from hard evergreen leaves, Cactus-stems, &c., which are 
covered by a thick cuticular coating. It may be assumed that in plants provided with 
a thick cuticle transpiration takes place principally through the stomata, and is therefore 
dependent on their smaller or larger number and size. The evaporation does not in 
this case proceed from the surface of the organ (or only to an imperceptible extent) 
but in its interior, viz. at the places where the cells of the parenchyma bound the inter- 
cellular spaces. These spaces may be supposed to be always at least nearly saturated 
with aqueous vapour ; but the vapour will escape through the stomata with every increase 
of its tension or decrease of the tension of the vapour without, and will thus give rise to 
the production of more vapour in the inside. The production of vapour in the inter- 
cellular spaces is moreover the more abundant the larger they are themselves, or in 
other words the larger the superficies of cell-wall which bounds them. This circum- 
stance, and the much larger number of stomata on the under side of the leaves, are 
clearly the reason why evaporation is generally so much more copious from it than from 
the upper side. Since water containing any substance in solution evaporates more slowly 
than pure water, and the more slowly the more concentrated and denser the solution, 
this must also be considered among the conditions which limit the transpiration of water 
from the sap of plants. It must not however be forgotten that evaporation takes place 
only on the external surfaces of the cell-walls of tissues, which on their part remove the 
water by imbibition from the cell-sap. 
The conditions now named which regulate transpiration are combined in the most 
various ways, and not only cause different plants to show different amounts of transpira- 
tion, but also the amount to be very difforent in the same plant at different times. 
A definite statement cannot however be made of the total amount of transpiration, i. e. 
of the quantity of water required by a plant during its period of vegetation, although 
certain very variable limits can always be assigned to each species in this respect. 
Two plants of the same species may, as any one may see, thrive equally well if one 
grows in damp soil and dry air, the other in dry soil and damp air, the former thus 
using up a large, the latter a small amount of water. In general the conditions of 
transpiration which have been mentioned exhibit periodic variations related to the 
meteorological distinction of day and night ; the temperature, the moisture of the air, 
and light, are usually favourable to evaporation by day, unfavourable by night ; but 
under certain circumstances this condition may even be reversed. 
ic) Currents of Water in the Wood. Superficial cells or those which bound intercel- 
lular spaces and lose water directly by evaporation would very soon collapse and dry up 
if they were not able again to replace that vi^hich they have lost. This can only take 
place by the flow of water from the adjoining cellular tissue from which no evaporation 
occurs ; but when this tissue is placed in the same condition as the former, it must also 
compensate its loss from more distant layers of tissue, and these again from those which 
are connected with the conducting organs or woody bundles which convey the water 
from the roots. The question here presents itself whether this movement of water 
within the succulent tissue (especially in the parenchyma of the leaves) is caused by 
endosmose from cell to cell, or whether it does not occur at least principally along 
the cell- walls, these latter forming the channels of communication between the woody 
bundles and the surfaces where the evaporation takes place, the contents of the cells 
being only incidentally concerned in the process. 
The chief evidence of the fact that the rapid currents of water in the roots, stem, 
and branches caused by transpiration take place only in the wood, i.e. in the lignified 
^ [It may be effected in the summer by means of the lenticels; see ante, p. 108.] 
