694 
MOLECULAR FORCES IN THE PLANT. 
stomata than those which require a less active interchange in consequence of slower 
growth and assimilation. In addition to this, organs with a thin cuticle are better 
adapted to bring about interchange of gas by diffusion than those whose epidermis 
is provided with a thicker cuticle which hinders the diffusion-current. This is clearly 
the reason why roots require no stomata, since, in consequence of their slow increase in 
size and their thin-walled slightly cuticularised epidermis, they can accomplish the 
interchange of oxygen and carbon dioxide by diffusion alone ; while the leaves, in 
consequence of their thick cuticle, require a large number of stomata in order rapidly to 
interchange large volumes of carbon dioxide with as large volumes of oxygen in sunshine. 
Even flowers and rapidly growing parasites which contain no chlorophyll possess stomata, 
though in smaller numbers, because they absorb a quantity of oxygen and exhale carbon 
dioxide. When the epidermis is replaced in the older parts of stems and roots by cork- 
periderm, the parts are not only externally impervious to air (with the exception of 
occasional fissures) in the ordinary sense, but even the interchange of gas by external 
diffusion practically ceases. But this case occurs only in those parts of plants where the 
fibro-vascular bundles form air-conducting vessels and usually also air-conducting wood- 
cells, by means of which an interchange of gas is brought about internally with that 
contained in the parenchyma enveloped by the cork. This is especially the case with 
woody Dicotyledons and Conifers. 
These considerations apply also to a great extent to aqueous vapour. The evapora- 
tion of the M^ater of vegetation, resulting, as we have seen in the previous paragraph, 
in the production of currents in the plant, is almost entirely prevented by cork-periderm 
and bark, and at least very much hindered by cuticularised epidermal cells. Since the 
parts of plants exposed to the air are covered with one or other of these epidermal 
structures, evaporation can in general only take place to a subsidiary extent from their 
surface ; the greater part of the aqueous vapour which these parts of the plant lose is 
evidently given off from the moist cell-walls in the interior of the tissue where they 
adjoin intercellular spaces and larger air-cavities. If these spaces are saturated with 
aqueous vapour, evaporation ceases ; but if the external air is comparatively dry the 
vapour escapes through the stomata, and evaporation into the intercellular spaces re- 
commences. If the transpiring tissue is heated, as by sunshine, the formation of vapour 
proceeds more rapidly in the interior, and the greater tension of the vapour causes its 
more rapid passage through the intercellular spaces and stomata. 
The surfaces of the organs of plants which are constantly in contact with water 
cannot exhale aqueous vapour through such fine openings as the stomata under the 
existing conditions of temperature ; stomata are therefore wanting in submerged plants, 
or occur only occasionally. The leaves, for instance, of Water-Lilies, which float on 
the water, are especially instructive in this respect; on the side in contact with the 
water they have no stomata or very few, on the upper side exposed to the air a large 
number. This is the more striking since leaves entirely exposed to the air have generally 
a lai-ger number of stomata on the under than on the upper side, where they are some- 
times entirely wanting. 
