132 PLANT STRUCTURES 
to the soil only intermittently in the form of rain, and 
often in sparing quantity, the plant envelops itself 
from end to end of its exposed portions in a water- 
proof cuticle; the only openings in its surface layer 
are the spongy tips of the root hairs on the one hand, 
and in the stomata on the other. These minutest of 
openings—so small that the number on a square inch 
of leaf surface often far exceeds a hundred thousand— 
might prove danger-points were they not most jeal- 
ously watched over. But each is provided with a pair 
of guard-cells ready to close the opening at any 
moment; and where drought threatens, the whole of 
the stomata are found in concealed positions. An 
ample pipe-system extends from root, through stem, 
to leaf, but it does not communicate directly with the 
openings at either end. All material, whether liquid 
or gaseous, absorbed or given out, has to run the 
gauntlet of the living cells, which are jealous watch- 
men, and allow only selected substances to pass 
through them. The crude building materials and 
food materials are assembled in the leaves, where in 
cells spread out to the light the chlorophyll is massed. 
Under the microscope, the chlorophyll is seen to be 
located in minute granules embedded in the semi- 
fluid contents of the cells. Well may we gaze in 
wonder at these tiny green specks. Each is so small 
that although a couple of hundred of them are often 
present in each cell, they occupy but a very small pro- 
portion of its volume. The cells themselves are of 
microscopic size. The chlorophyll itself occupies only 
quite a small portion of the corpuscle in which it is 
immersed; yet on its activity as spread in this infini- 
tesimal quantity through the leaves the whole organic 
