282 
THE LEAF 
249. Excretion. — Excretion in plants is the same as in 
animals in this respect that it is getting rid of material or sub¬ 
stances which are no longer of use to the plant. It very often 
happens, however, that these are not removed from the 
plant as in the case of animals, but are simply stored where 
they can do no harm. An example of this is the crystals 
of calcium oxalate which give to sorrel, or sour grass, its 
sour taste. The line between excretion and the secretion of 
certain substances, such as the oils in peppermint plants, is 
not easily drawn. In the case of the secretions, however, 
they often serve the purpose of preventing animals from 
eating the plants. 
250. Transpiration. — This is the process of evaporation 
which takes place in plants, water in the form of vapor escap¬ 
ing through very small openings in the leaves called stomata. 
It is not one of the vital processes but closely connected 
with them. Its only similarity to perspiration in animals, 
to which it has sometimes been likened, is that water is 
given off through openings in the outer covering of the plant. 
Transpiration is unavoidable because the roots of a plant 
usually take up more water than is needed for the vital 
processes, and because this excess accumulates in spaces 
which communicate with the outside through the stomata. 
The function of the stomata is to allow air containing ni¬ 
trogen to pass into the leaf, and excess oxygen to pass out. 
Incidentally, however, water passes out too. When transpi¬ 
ration is too rapid, the plant is deprived of needed water. 
Since transpiration is a menace to the well-being of a plant, 
numerous devices have been developed for regulating it. 
Thus: 1. The surface of a leaf is covered above and below 
by a layer of cutin, a transparent substance impervious to 
water, perforated only where stomata occur. 
2. The stomata are on the under side of the leaves in most 
plants, this position being less favorable to transpiration 
than the upper side. 
