88 A GLIMPSE OF THE LIFE OF CHLOROPHYLLOUS PLANTS. 
epidermis of its foliage leaves which envelops and protects 
the tissue which contains the chlorophyll. The surface, or 
epidermis, of a leaf, and in the majority of plants on the 
under surface of the leaf, has innumerable openings commu- 
nicating directly with the external air. 
It is particularly interesting to note that in the cells of 
the epidermis only those guarding the stomata contain 
chlorophyll grains. 
These stomata have most important functions to perform ; 
they allow, and, also prevent, the exit of water which has 
come through the plant from the root; they permit the 
carbon dioxide of the air to reach the chlorophyll of the 
assimilating cells of the leaf, and they allow the oxygen gas 
to escape which results from the decomposition of carbon 
dioxide by the chlorophyll. 
It is supposed, and, I believe correctly, that this open- 
ing and closing of the stomata is to be ascribed to the 
presence of chlorophyll-grains in the guard cells to which 
I have referred. These chlorophyll grains induce assimi- 
lation within the guard cells, and consequently produce in 
them a change of shape to suit the requirements. All green 
coloured portions of a plant contain cells which are perme- 
ated with chlorophyll grains, and it is the function of these 
grains to assimilate carbon from the carbon-dioxide of the 
air. During the sunshine of the day, other conditions 
being suitable, the guard cells of the stomata change their 
shape in such a manner as to open the aperture in order 
that transpiration and assimilation can progress more 
rapidly, the reverse being the case at night. 
As far as human power is at present capable of dis- 
cerning, the first assimilated or manufactured product in a 
plant is starch, composed of carbon and the elements of 
water, hydrogen and oxygen; if we had within our reach a 
test which would enable us to discern earlier, or interme- 
diate products, we would be in a position to say that they 
occur, but we are not at present in that position. 
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