LEAVES AND THEIR WORK 



123 



product is starch, and a certain by-product (corresponding to the 

 waste in a mill) is also given out. This by-product is oxygen. To 

 understand the process 

 fully, we must refer to a 

 small portion of the leaf. 

 Here we find that the cells 

 of the green layer of the 

 leaf, under the upper epi- 

 dermis, perform most of 

 the work. The carbon 

 dioxide is taken in through 

 the stomata and reaches 

 the green cells by way 

 of the intercellular spaces 

 and by diffusion from cell 

 to cell. Water reaches 

 the green cells through the 

 tracheal tubes of the veins. 

 It then passes into the cells Diagram to illustrate the *>nnation of starch - 

 by osmosis, and there becomes part of the cell sap. The light 

 of the sun easily penetrates to the cells of the palisade layer, 



- 



Diagram (after Stevens) to illustrate the processes of breathing, food-making, 

 and transpiration which may take place in the cells of a green leaf in the sunlight. 



