76 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, hy a series of steps, recombined by the chlorophyll 



J 



