4 o METABOLISM 



presents special peculiarities. On the outside, the lumen is constricted by 

 horn-like ridges (H) ; then follows an enlargement known as the vestibule of 

 the stoma. The vestibule again narrows to form the slit proper, followed by a 

 further widening inwardly the rear vestibule once more contracted by a second 

 pair of ridges. The inner contour does not, however, run parallel with the outer 

 one, but forms almost a half circle. The concave wall is thus not of the same thick- 

 ness throughout ; it is relatively thin in the middle and thicker above and below 

 where the ridges are (see Fig. 8, /). The occurrence of abundant chlorophyll 

 in the guard-cells must be specially noted, the ordinary epidermal cells being 

 as a rule destitute of green pigment. Further, the protoplasm of the guard 

 cell encloses a large vacuole which is the seat of great osmotic activity. Under 

 the influence of osmotic pressure the cell-walls are stretched, but the concave 

 sides, owing to their greater thickness, are more able to resist this extension 

 than are the convex sides, so that the stretching is more obvious on the latter 

 than the former. The effect of this differential stretching is best illustrated 

 by a model. If one takes a caoutchouc tube, completely closed, and having 

 a strengthening layer pasted along one side, and forces into it air or water, 

 though straight at first, it becomes bent (Fig. 8, V, VI). Now imagine two 

 such tubes with the strengthened sides facing each other, united by their ends, 

 but with the central region free ; if turgor be produced in these tubes they will be 

 seen to separate from each other in the middle. Much the same sort of thing 

 takes place in the guard-cells of the stoma ; increase of pressure induces the 

 slit to open, as much as is shown in comparing Fig. 8, /// and IV. We speak of 

 the stoma in the first case as closed, in the second as open; as a matter of fact, 

 however, when turgor is at a minimum, the two guard-cells do not lie so closely 

 together as to hermetically seal the opening, although it is sufficiently obliterated 

 to make the amount of water vapour which passes through scarcely worth speak- 

 ing about ; in other words, stomatal transpiration is almost as good as sup- 

 pressed. By altering the pressure in the guard cells the plant is able to vary 

 the width of the slit very considerably, and in this way the stomata become 

 regulators of transpiration to a degree which would scarcely be anticipated. 



The width of the slit does not depend entirely on the osmotic pressure in 

 the guard-cells, but is also influenced by pressure in the cells surrounding 

 them. If this pressure be eliminated, e.g. by pricking these cells, it will be 

 seen that the stoma opens at once, although the pressure in the guard cells 

 is not at the same time increased. Conversely also, increase in turgescence 

 of the surrounding epidermal cells may bring about a passive closure of the 

 stoma. Authorities differ in their estimates of the degree to which this activity 

 in the general epidermal cells is functional in nature. SCHWENDENER (1881) 

 held that it was of no importance, LEITGEB (1886) believed that it played a 

 great part in the process, while DARWIN (1898) takes an intermediate position. 

 So far as we are concerned we may take it that the active movements of the 

 guard-cells alone are quite sufficient to account for the opening and closing of 

 the slit. The effect of variations in pressure within the guard-cells may be 

 readily demonstrated under the microscope. If the preparation shows the 

 stomata open, addition of a plasmolytic solution will quickly cause them to 

 close, and they may be made to open again at once by replacing the plasmolytic 

 solution with water. In nature, variations in pressure so extreme as to lead 

 to complete abolition of turgor, such as is effected by plasmolysis, do not 

 occur ; on the contrary a pressure of several atmospheres is maintained even 

 when the stoma is quite closed. 



The conditions under which opening and closing of the stomata take place 

 are very different in different plants, still, in general, we may say that stomata 

 in appropriate ways regulate the amount of transpiration, and that the plant 

 is thus able to protect itself from wilting. Conditions connected with this 



