MOVEMENTS OF GASES IN PLANTS. 6 1 'J 



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 evapor- 

 ation of the water 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. 



Those 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 ex- 

 isting 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 or very few stomata, 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 larger number of stomata on the under than on the upper side, where they are some- 

 times entirely wanting. 



