PLANTS MAKE FOOD 



93 



understand the process fully, we must refer to a small portion 

 of the leaf shown below. Here we find that the cells of the green 

 layer of the leaf, under the upper epidermis, 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 osmosis from cell to cell. Water reaches the green cells 

 through the veins. It then passes into the cells by osmosis, and 

 there becomes part of the cell sap. The light of the sun easily 

 penetrates to the cells of the palisade layer, giving the energy 



LIGHT 



I 



Soil water 



Diagram (after Stevens) to illustrate the processes of breathing and food 

 making in the cells of a green leaf in the sunlight. 



needed to make the starch. This whole process is a very delicate 

 one, and will take place only when external conditions are favorable. 

 For example, too much heat or too little heat stops starch making 

 in the leaf. This building up of food and the release of oxygen 

 by the plant in the presence of sunlight is called photosynthesis. 



Manufacture of Fats. — Inasmuch as tiny droplets of oil are 

 found inside the chlorophyll bodies in the leaf, we believe that fats, 

 too, are made there, probably by a transformation of the starch 

 already manufactured. 



Protein Making and its Relation to the Making of Living Matter. 

 — Protein material is a food which is necessary to form protoplasm. 



