94 VEGETABLE PHYSIOLOGY 
curved so that the orifice is widely open. This is helped 
by 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 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 Lower surface 
Mezereon 5 ‘ . pone 4,000 
Peony . ‘i ‘; none 13,790 
Vine ‘ . none 13,600 
Olive .. : none 57,600 
Holly : none 63,600 
Laurustinus none 90,000 
Cherry-laurel none 90,000 
Lilac ‘ none 160,000 
Hydrangea none 160,000 
Mistletoe . 200 200 
Tradescantia 2,000 2,000 
House-leek 10,710 * 6,000 
Garden Flag . 11,500 11,500 
Aloe ‘ . 25,000 20,000 
Yucea. 4 . . 40,000 40,000 
Clove Pink. ‘ i * . 388,500 38,500 
The modification of the turgescence of the guard-cells 
is caused by the osmotic transference of water between 
them and the other cells of the epidermis from which they 
are separated by thin walls. The vapour which is in the 
intercellular space below them does not penetrate them, 
the walls abutting on the space being thick and cuti- 
cularised. The osmosis alluded to may be associated with 
the presence of the chloroplasts in the guard-cells, which 
are instrumental in the production there of various sub- 
stances, so that their contents have a higher osmotic 
equivalent than those of the epidermal cells which are con- 
