70 THE FLOWERING PLANT. 
The quantity of water thus lost depends not only upon the num- 
ber of stomata present, but also upon their condition. That is to 
say, a stoma is not always open, and when it is, the aperture is 
not always of the same size. Stomata generally close at night or 
on wet days, and open in sunny weather. This is due to the 
guard-cells. Their action is complicated, but the main principle 
is easily understood. The fact that these cells always contain 
chlorophyll granules now receives its explanation. In the sun 
these granules begin active assimilation, and this causes crude 
sap to diffuse into the guard-cells, which thus become extremely 
turgid and increase in size. Curved cells, like the ones we are 
dealing with, naturally become more curved when they increase in 
size. In the case of the guard-cells this means that the stoma 
opens. But this is not all. The parts of the walls of the guard- 
cells which face outwards and inwards (2.e., towards the outside 
and inside of the leaf) are very much thickened, and at night or 
in the wet, when the cells are not very turgid, act like springs, 
which tend to flatten the cells in the plane of the leaf-surface. 
Since the walls of the guard-cells are very thin and flexible where 
they face one another, this flattening causes them to bulge into 
the slit of the stoma, making it smaller, or even closing it. 
When the guard-cells are very turgid the outer and inner spring- 
like parts of their walls are forced away from one another, while 
their thin parts are easily pulled into a position more or less 
vertical to the surface of the leaf, thus opening the stoma. The 
walls of the intercellular spaces with which the stomata com- 
municate present a very large surface, from which the evaporation 
of water can take place. 
The importance of transpiration to land plants is seen from 
the fact that where it is checked by an over-damp atmosphere 
sickliness invariably ensues. On the other hand, excessive trans- 
piration is harmful. This is often observed in the case of plants 
grown in the hot air of a room, Even when abundantly watered, 
many of them droop and die, because the roots are unable to 
absorb rapidly enough to keep pace with the evaporation from 
the leaves. This is particularly the case when the plants are 
herbaceous in texture. Protection from too vigorous trans- 
piration may be afforded by a condensed form presenting rela- 
tively little surface, or by much thickened and cuticularized cell- 
walls (aloe, &c.), or again by the assumption of a vertical position, 
the best example of which is found in the phyllodes mentioned 
on p. 53. Transpiration is of importance for two chief reasons. 
In the first place, young shoots contain go per cent. or more of 
water, without an abundant supply of which they cannot be 
formed. Such a supply would be out of the question if it were 
