Page:First course in biology (IA firstcourseinbio00bailrich).pdf/120

sunlight. Fig. 114 shows the structure. There are two guard-cells at the mouth of each stomate, which may in most cases open or close the passage as the conditions of the atmosphere may require. The guard-cells contain chlorophyll. In Fig. 115 is shown a case in which there are compound guard-cells, that of ivy. On the margins of certain leaves, as of fuchsia, impatiens, cabbage, are openings known as water-pores.

Fig. 114.—Diagram of Stomate of Iris (Osterhout).

Fig. 115.—Stomate of Ivy, showing compound guard-cells.

Stomates are very numerous, as will be seen from the numbers showing the pores to each square inch of leaf surface:

                    Lower surface Upper surface
Peony 13,790 None
Holly 63,600 None
Lilac 160,000 None
Mistletoe 200 200
Tradescantia 2,000 2,000
Garden Flag (iris) 11,572 11,572

The arrangement of stomates on the leaf differs with each kind of plant. Fig. 116 shows stomates and also the outlines of contiguous epidermal cells.

Fig. 116.—Stomates of Geranium Leaf.

The function or work of the stomates is to regulate the passage of gases into and out of the plant. The directly active organs or parts are guard-cells, on either side the opening. One method of opening is as follows: The