TRANSPIRATION 99 
in contact with the other epidermal cells, however, re- 
mains thin. When the guard-cells are full of water, 
their form and mode of attachment cause them to become 
curved go that the orifice is widely open. This is helped 
hy the thickening of the free edges, which makes it difficult 
for them to swell in the direction of each other. When, 
on the other hand, they lose their water, they relax, and 
their edges coming into contact, the aperture between them 
is more or less completely closed (fig. 66). 
The edges of the guard-cells when viewed in section are 
slightly convex (fig. 67). The turgor which results from 
the imbibition of water stretches them in the vertical direc- 
tion as well as in the horizontal one. This tends to lessen 
the vertical convexity, and at the same time to cause a 
considerable vertical tension. When the escape of water 
relieves this tension the thickened upper corners of the 
cell recoil, lessening the vertical diameter of each and 
increasing this convexity, sometimes bringing the con- 
vexities of the two cells into contact with each other, and 
so completely closing the aperture. 
The number of the stomata varies very considerably. 
The following table will give some idea of their abundance 
in leaves, and it will be observed that the number of stomata 
is usually greatest in those leaves from whose upper surface 
they are entirely absent. 
Stomata in one Square Inch of Surface 
Upper Surface I.ower Surface 
Mezereon . zi ‘ 5 none 4,000 
Peony a“ ; : : 2 none 13,790 
Vine . 7 : - : none 13,600 
Olive . : é 2 i none 57,600 
Holly 5 : ; : none 63,600 
Laurustinus , F : F none 90,000 
Cherry-laurel ‘ : : none 90,000 
Lilac . : : d : : none 160,000 
Hydrangea 3 i ; : none 160,000 
Mistletoe . : : 4 j 200 200 
Tradescantia 7 ‘ : r 2,000 2,000 
House-leek ; ‘ F . 10,710 6,000 
Garden Flag : F ‘ . 11,500 11,500 
Aloe . A ‘i : ‘ . 25,000 20,000 
Yucca ‘i . 4 . . 40,000 40,000 
Clove Pink i 5 ; . 38,500 38,500 
