EPIDERMIS OF IRIS. 83 



In this way a number of sections are prepared for further use, 

 and they can be laid in the meantime, by means of a camel-hair 

 brush, in a watch-glass filled with water. Place some of the 

 sections in water under the microscope, and they will show, in 

 favourable places, median cuts through the stomata, as shown in 

 Fig. 28, B. As such a cross-section will show, the epidermal 

 cells of Iris florentina are more strongly thickened on their outer 

 than on their inner side. The inner walls, however, are also 

 pretty thick, while the radial walls are only slightly thickened. 

 This structure is connected with the function of the epidermis, 

 which not only has to serve as an outer protecting sheath, but 

 also has to serve as a water-reservoir. The thin radial, walls 

 easily allow a change in the capacity of the cells, which, by 

 means of a bellows-like play, diminish in height with loss of 

 water, and enlarge again with increase of water. The two guard- 

 cells lie recessed between the epidermal cells ; the manner in 

 which the latter overlap the guard-cells can be at once seen. 

 The stomatic-pit leads down to the guard-cells. These latter are 

 strongly thickened on the upper and under side. Above the cleft 

 is found a special thickening ridge which in cross-section con- 

 stitutes a beak-like projection. Under it the wall is for a short 

 distance thinner, and the same over a broader area on the other 

 side, towards the adjoining epidermal cell. This thickening of 

 the wall is connected with the mechanism of movement of the 

 guard-cells, which would become higher and curve more strongly, 

 thus widening the cleft, when their turgidity increases ; but their 

 curvature would diminish, they would flatten, and thus narrow 

 the cleft, when their turgidity decreases. In order that the 

 movement of the guard-cells may not be hindered, the strongly- 

 thickened wall of the surrounding epidermal cells thins suddenly 

 at their line of union with the guard-cells ; the guard cells are 

 here attached as with hinges the stomatic hinges. Under the 

 stoma is found an air-cavity, the ventilating chamber (a), a 

 large intercellular space, under natural conditions filled with air, 

 surrounded by chlorophyll-containing cells, and connected with 

 the intercellular spaces which are found between them. A cross- 

 section laid in chlorzinc iodine shows us that the walls of the 

 epidermal cells stain violet in their entire extent, with the exception 

 of a thin outer layer, somewhat wrinkled, the so-called Cuticle (c), 

 which becomes yellowish- brown. This cuticle swells out at the 

 stoma into the beak-like projection which we have already men- 



