82 PLANT PHYSIOLOGY AND ECOLOGY 



reaches the leaves, and consequently the chloroplasts. The carbon 

 dioxide required is absorbed by the leaves through the stomata. 

 With the air it enters the chamber below the guard-cells, and then 

 finds its way throughout the leaf by means of the air spaces. 

 The degree of photosynthetic activity depends in a large measure 

 upon the rapidity with which carbon dioxide enters the leaf and 

 diffuses through it. This is due to the fact that, while the suppl}^ 

 of this gas is practically unlimited, the amount actually present 

 in the air is very small, usually about 0.04% by weight. In con- 

 sequence aeration, i.e., the movement of gases into and through the 

 leaf, becomes a very important function of the latter. It depends 

 in the first place upon the number of the stomata, the size of 

 their pores, and the length of time they are open. The advantage 

 of having the stomata open in the light thus becomes evident, as 

 well as that of placing them chiefly upon the lower surface, or of 

 sinking them, so that the chances of water loss closing them are 

 reduced. Ease of diffusion within the leaf depends upon the 

 thickness of the leaf and the number and size of the air spaces. 

 The rapidity of diffusion is chiefly determined by the rate at which 

 the carbon dioxide is being used. Thus, active sun leaves absorb 

 and use much more carbon dioxide than the less active shade leaves, 

 although the latter regularly possess larger air spaces. Gases 

 penetrate the walls of the chlorenchym cells only in solution. This 

 is brought about by the cell-sap which fills the wall, and in 

 consequence carbon dioxide and water reach the chloroplast 

 together. 



103. Chemical changes during photosynthesis. The precise 

 changes that take place during photosynthesis are in doubt. It 

 is definitely known that the carbon dioxide and water are broken 

 down, and that oxygen is set free and escapes from the leaf. In 

 fact, the evolution of oxygen from the plant is the usual test of 

 photosynthetic activity. The first visible product of the latter 

 is the starch formed in the chloroplast. It is evident, however, 

 that a number of changes must occur before starch can appear. 

 The number and nature of these changes are not known with 

 any degree of certainty. The formation of grape sugar, or glu- 

 cose, precedes that of starch, and it is probable that glucose is 

 formed from a simpler carbohydrate, formaldehyde. The prob- 

 able stages in photosynthesis may be indicated by the follow- 

 ing series of formulse. 



