THE LIFE OF A PRIMROSE. l6l 
the active protoplasm is made of these three added 
to a fourth, nitrogen, and it also contains phosphorus 
and sulphur. 
And so hour .after hour and day after day our 
primrose goes on pumping up water and ammonia 
from its roots to its leaves, drinking in carbonic acid 
from the air, and using the sun-waves to work them 
all up into food to be sent to all parts of its body. 
In this way these leaves act, you see, as the stomach 
of the plant, and digest its food. 
Sometimes more water is drawn up into the leaves 
than can be used, and then the leaf opens thousands 
of little mouths in the skin of its under surface, which 
let the drops out just as drops of perspiration ooze 
through our skin when we are over- 
heated. These little mouths, which 
are called stomates (a, Fig. 42) are 
made of two flattened cells, fitting 
against each other. When the air 
is damp and the plant has too 
much water these lie open and let 
it out, but when the air is dry, and . f 
J ' Stomates of a leaf. 
the plant wants to keep as much 
water as it can, then they are closely shut. There 
are as many as a hundred thousand of these mouths 
under one apple-leaf, so you may imagine how small 
they often are. 
Plants which only live one year, such as migno- 
nette, the sweet pea, and the poppy, take in just enough 
food to supply their daily wants and to make the 
seeds we shall speak of presently. Then, as soon as 
their seeds are ripe their roots begin to shrivel, and 
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