118 a BOTANY 
to a less degree in the upper, appear small openings, 
the stomata, through which the leaf receives air and 
gives out oxygen and water. The whole structure 
of the leaf is admirably adapted for the ingress and 
egress of atmospheric gases. In the loose spongy 
tissue that forms the lower part of the mesophyll 
there are large spaces communicating, through the 
stomata, with the air outside. The more compact 
upper part of the mesophyll is palisade tissue consisting 
of elongated cells, which by their arrangement are 
able, when necessary, to protect the chlorophyll from 
the effects of too strong light. All the cells of the 
mesophyll as well as the guard cells, which form the 
entrance to the stomata, contain chlorophyll. This 
may be dissolved out by methylated spirit or other 
alcohol, in which ease a green solution is obtained and 
the leaf becomes colourless. 
The stomata (Fig. 81) are so numerous that, by 
diffusion, they admit air to the mesophyll as freely 
as if there were no epidermis present. This air comes 
into contact with the walls of the interior cells, the 
protoplasm of which, exerting its selective influence, 
permits the carbon dioxide alone to. pass through. 
Inside the cell, then, we have the chlorophyll, which 
ean absorb and utilise the energy of the sunlight, the 
carbon dioxide which has penetrated from the air, and 
the water and dissolved mineral salts brought from 
the soil. It will now be necessary to consider the 
processes that are collectively known as photosynthesis. 
PHOTOSYNTHESIS. 
Photosynthesis (Gk. photos light and synthesis a 
putting together) or carbon-assimilation, as it is 
sometimes called, is a process that comprises at least 
two stages. In the first place the protoplasm of the 
mesophyll cells, employing the energy of the sunlight 
absorbed by the chlorophyll, decomposes the carbon 
