86 VI. THE EPIDERMIS STOMATA. 



thicker sections, however, serve for general information. The 

 epidermal cells on both sides of the leaf are distinguished, as 

 cross-sections show, by their considerable height ; those on the 

 upper side especially are so high that they alone form half the 

 thickness of the leaf. Many of these epidermal cells are seen to 

 be divided by cross walls. The leaves of Tradescantia possess, 

 therefore, in their epidermis, a specially efficient water- reservoir. 

 The accessory cells of the stomata are, as the cross-section shows, 

 quite flat and thin, so that a great air-chamber, of the depth of the 

 surrounding epidermal cells, is formed under the stoniatic apparatus. 



Stomata of Lilium. Very beautiful large stomatic apparatus 

 are present on the under side, more rarely the upper side, of the 

 leaves of the white garden lily, Lilium candidum, and this can 

 therefore be recommended as an object for investigation. The 

 epidermal cells are elongated in the long axis of the leaf, lie in 

 straight rows, but have, however, an undulating outline. The 

 stomatic apparatus stand as prolongations of the epidermal 

 cells, and at the same height with them. The cross-section is 

 easy to obtain, and shows a hinge at the point of junction of the 

 guard-cells in the form of a sudden thinning of the strongly 

 thickened outer wall of the neighbouring epidermal cells. 



Epidermis of Aloe. The species of Aloe and Agave possess 

 epidermal cells thickened very strongly on their outer sides, and 

 stomata proportionally deeply sunk in the epidermis. Because 

 it is specially instructive, and not difficult to prepare, we select 

 for observation Aloe nigricans, a greenhouse plant with ligulate 

 leaves arranged in two series (ranks) ; but other species of Aloe 

 can serve as substitutes. In surface-sections, the epidermis of 

 upper as well as under side appears formed of regular polygonal 

 cells, mostly hexagonal. The cavity (or lumen) of each of these 

 cells is reduced to a relatively small, rounded space, which ap- 

 pears dark, because in cutting it has become filled with air. 

 The stomata are found on both sides of the leaf ; deep pits lead 

 up to them. These stomatic pits are always bounded by four 

 cells, and have a rectangular contour ; a somewhat prominent 

 rim surrounds the pit. If you wish to see the guard-cells, it 

 suffices to lay the sections on the glass-slide with their inner 

 side upwards. The guard-cells are comparatively broad and 

 short ; amongst their contents are noticeable strongly refractive 

 spherical oil-globules. As the epidermis is very hard, the cross- 

 section is best taken between two pieces of bottle cork. The 



