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) Veins. 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 upon it. 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- 

 tinually. 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. 



