18 A TEXT-BOOK OF BOTANY 
are irregular in form, and so loosely arranged as to leave 
air-spaces between the cells, the whole region forming the 
spongy tissue (Fig. 12). The air-spaces communicate with 
one another, thus forming a labyrinthine system of air- 
chambers throughout the spongy mesophyll. It is into 
this system of air-chambers that the stomata open, and 
thus what may be called an internal atmosphere is in con- 
tact with all the green working cells, and this internal 
atmosphere is in free communication through the stomata 
with the external atmosphere. The significance of the 
palisade arrangement will be considered under the head of 
leaf protection. 
(3) Verns.—In the cross-section of the leaf there will 
be seen also here and there, embedded in the mesophyll, 
the cross-sections of veins and veinlets, that constitute the 
supporting framework of the leaf and conduct material to 
and from the green working cells (Fig. 12). 
14. Photosynthesis.—The peculiar work of green plants 
or green parts of plants is to manufacture the kind of food 
best known as sugars and starch, such foods being called 
carbohydrates. This manufacture is exceedingly important, 
for all life is dependent uponit. If green plants should stop. 
the manufacture of carbohydrates, the food supply of the 
world would soon be exhausted. All other forms of food 
are derived from carbohydrates in some way, and only green 
plants can add to the stock that is being drawn upon con- 
Jnually. This means that green plants must manufacture 
carbohydrates not only for their own use, but also for the 
use of animals and of plants that are not green. Since 
leaves are chiefly expansions of green tissue, they are con- 
spicuous in the manufacture of carbohydrates; but it must 
be remembered that the manufacture goes on wherever 
there is green tissue, whether it is found in leaves or not. 
A very conspicuous fact about this manufacture is that 
it cannot go on unless the green tissue is exposed to light. 
