X] GUARD-CELLS 101 



rain- and dew-drops, &c., and the same is true of the rolled 

 leaves of the Ericaceae, e.g. Heaths, Ling, &c. 



There seems to be no doubt that the number of 

 stomata per sq. mm. varies according to circumstances. 

 Leaves developed in full sun have more than those de- 

 veloped in shade. On shoot-axes the stomata may be 

 isolated and far apart, though leaves of the same shoot 

 may have large numbers per sq. mm. 



A typical stoma, seen in plan from the outside, shows 

 the two guard-cells looking like a pair of curved sausages 

 joined at their ends, and concave towards the slit-shaped 

 aperture between them. In vertical section across the 

 middle of the long axis of the stoma, the guard-cells look 

 like two boxes with thin lateral walls, and with a thickened 

 roof and floor. The thin lateral walls bounding the aper- 

 ture are usually bulging towards one another, and their 

 mutual advance towards contact closes the aperture more 

 and more, while their recession opens it. It commonly 

 happens that a stiff rim or ridge of the thickened roof 

 overhangs the bulging wall, and there is often a similar 

 ridge from the floor below. A fine hair passed through 

 the aperture would project freely into a large intercellular 

 space or sub-stomatal cavity, which communicates with 

 the labyrinths of intercellular spaces of the tissues beneath, 

 and we have shown already (pp. 91 96) what kinds of 

 experiences would be in store for a minute organism 

 which entered one of these. 



The development of a typical stoma occurs as follows. 

 While the epidermis is still young, a small cell is cut off 

 by a partition-wall from one of the epidermal cells, and 

 vertical to the plane of the latter ; either from its end as 

 in the prismatic cells of many Monocotyledons, or out of a 

 corner, as in the tabular cells of most Dicotyledons, or 

 even right out of its area by a more or less oval ring-like 



