In the centre of the mouths may be seen a dark spot, which is the aperture through which the air communicates with the passages between the cells in the interior of the structure. In the flowering plants their shape is generally rounded, though they sometimes take a squared form, and they regularly occur at the meeting of several surface cells. The two kidney-shaped cells which form the “mouth” are the “guard-cells,” so called from their function, since, by their change of form, they cause the mouth to open or shut, according to the needs of the plant. In young plants these guard-cells are very little below the surface of the leaf or skin, but in others they are sunk quite beneath the layer of cells forming the outer coat of the tissue. There are other cases where they are slightly elevated above the surface. Stomata are found chiefly in the green portions of plants, and are most plentiful on the under side of leaves. It is, however, worthy of notice, that when an aquatic leaf floats on the water, the mouths are only to be found on the upper surface. These curious and interesting objects are to be seen in many structures where we should hardly think of looking for them; for instance, they may be found existing on the delicate skin which envelops the kernel of the common walnut. As might be expected, their dimensions vary with the character of the leaf on which they exist, being large upon the soft and pulpy leaves, and smaller upon those of a hard and leathery consistence. The reader will find ample amusement, and will gain great practical knowledge of the subject, by taking a plant, say a tuft of groundsel, and stripping off portions of the external skin or “epidermis” from the leaf or stem, etc., so as to note the different sizes and shapes of the stomata. On the opposite bottom corner of Plate I. Fig. 25, is an example of a stoma taken from the outer skin of a gourd, and here given for the purpose of showing the curious manner in which the cells are arranged about the mouth, no less than seven cells being placed round the single mouth, and the others arranged in a partially circular form around them. Turning to Plate II., we find several other examples of stomata, the first of which (Fig. 1) is obtained from the under surface of the buttercup leaf, by stripping off the external skin, or “epidermis,” as it is scientifically termed. The reader will here notice the slightly waved outlines of the cell-walls, together with the abundant spots of chlorophyll with which the leaf is coloured. In this example the stomata appear open. Their closure or expansion depends chiefly on the state of the weather; and, as a general rule, they are open by day and closed at night.