7 6 
THE VEGETATIVE FUNCTIONS OF PLANTS 
are in direct connection with the outside air through the 
stomata. In the presence of sunlight the formation of 
carbohydrates begins in the green cells. This uses up 
the carbon dioxide in the cells, and the supply is renewed 
from the intercellular spaces. The gas passes through 
the cell-walls and the layer of protoplasm in solution in 
FIG. 56. A tree (hornbeam), seen from "outside," showing the 
dense foliage at or near the tips of the branches. The same tree as in 
Fig. 57- 
water. This results in reducing the amount (and thus 
the pressure) of this gas in the intercellular spaces, and as 
a result more carbon dioxide passes by diffusion through 
the stomata to the intercellular spaces. Thus, as fast 
as the gas is used the supply is renewed from without. 
76. Photosynthesis. Within the cell, the carbon di- 
oxide and water (or simple combinations of these) are 
finally, by a series of steps, recombined by the chlorophyll 
