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 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. 



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

 times entirely wanting. 



