280 TRANSPIRATION. 



but is shut off from the atmosphere by a firm cuticle through which water- vapour 

 can only penetrate with great difficulty, the aqueous vapour which is exhaled by 

 the branched and other cells of this parenchyma would saturate the lacunae, and 

 further evaporation would be thereby prevented. There must, therefore, be a 

 direct communication with the outer air surrounding the leaf; the epidermis of the 

 leaf must possess apertures through which the water- vapour can escape. The already 

 repeatedly mentioned stomata are to be looked upon as such apertures. 



Stomata arise in this way; in a particular epidermal cell a partition wall first of 

 all divides it into two cells. This cell-wall splits, and the cleft widens, forming 

 a short canal which pierces the epidermis, and constitutes a connection between the 

 outer air and the air-containing lacunae in the interior of the leaf. This short canal 

 is called the pore of the stoma, and the two cells which border it are termed guard 

 cells. These two cells regulate the outrush of aqueous vapour, i.e. of that vapour 

 which has been excreted by the thin- walled cells of the spongy parenchyma, and 

 passed into the adjoining passages in the interior of the leaf. That cavity which is 

 placed immediately beneath the narrow, short canal of the stoma, and is connected 

 by passages with other spaces further within the green tissue of the leaf, is termed 

 the respiratory cavity. 



The number of the stomata or transpiration-pores which pierce the epidermis of 

 the leaf varies very considerably. In the leaves of cabbages (Brassica oleracea) on 

 1 sq. mm. of the upper surface there are nearly 400, and on the under side over 700. 

 In the leaves of the olive-tree, on the same extent of surface of the under side, over 

 600. Succulent plants have remarkably few stomata. On 1 sq. mm. of the leaves 

 of the House-leek (Sempervivum tectorum) and of the yellow Stone-crop (Sedum 

 acre) only 10-20 are to be met with. In the majority of cases, on a similar extent 

 of surface, between 200 and 300 stomata are to be found. The under side of an 

 oak leaf, 50 sq. cms. in area, showed over two million stomata. They are in most 

 cases scattered fairly uniformly over the surface of the leaf; on the leaves of grasses 

 and pines, as well as on the green stalks of the horsetails, they form straight 

 regular rows which run longitudinally; on the leaves of some species of saxifrage 

 (Saxifraga sarmentosa, japonica, &c.) they appear crowded together in small 

 isolated groups; and on the leaves of the Begonia they are generally to be seen side 

 by side in pairs. Obviously they are principally developed just where the epidermis 

 overlies spongy parenchyma, and as in the majority of cases this parenchyma is 

 situated towards the under side of the leaf, the greater number of stomata are to be 

 found on this side. 



In most flat membraneous leaves, which have one side directed towards the sky 

 and one towards the earth, stomata are entirely wanting on the upper surface, 

 being restricted to the under side. An exception to this is afforded by the orbicular 

 flat leaves which float on the surface of water, e.g. those of the floating Pond-weed 

 (Potamogeton natans), of the Frogbit (Hydrocharis morsus-rance), and of the 

 water-lilies (Nymphcea, Nuphar, Victoria). These are covered with stomata on the 

 upper side, while on the lower side, which is in contact with the water, stomata are 



