LEAF FUNCTIONS 99 



We have also learned that this oxidation produces carbon di- 

 oxide which we throw off in respiration. Now we can see that the 

 plants use this discarded carbon dioxide for making their food, 

 and return to us the oxygen which is necessary for our life. 



This is a glimpse of one of the great " circles of nature." 



Other Leaf Functions. Starch making, while the most im- 

 portant, is not the only function of leaves. In their marvelous 

 chemical laboratory go on the processes of digestion, proteid manu- 

 facture, assimilation, respiration, and excretion of water (trans- 

 piration) . Digestion is necessary to put the food stuff into soluble 

 form so that it may act in osmosis and flow through the ducts. 

 As to proteid manufacture, little is known, except that the carbon, 

 hydrogen, and oxygen of the starch are combined with nitrogen, 

 sulphur, and phosphorus from the soil water in a way that we 

 cannot understand, much less imitate, and that proteids are the 

 result of the process. Assimilation is active in leaves and all other 

 living parts of the plant, since this is the process by which the 

 nutrients actually become part of the living protoplasm and tissue 

 of the organism. Respiration (oxidation) goes on wherever liv- 

 ing plant tissue is directly exposed to air; while less active than 

 in animals the process is just as essential, since it supplies the 

 energy which keeps the plant alive. Much extra water is absorbed 

 at times by the roots, in their transfer of nitrogen compounds and 

 mineral salts from the soil. The useful elements are used in food 

 making and the surplus water is passed off by way of the spongy 

 layer, air spaces, and stomata. This process is called transpiration 

 and differs from mere evaporation, in that the loss of water is 

 regulated by the stomata and so corresponds to the needs of the 

 plant. It does not depend upon the temperature alone, as does 

 evaporation. 



We find in the leaf the processes of food manufacture, diges- 

 tion, and assimilation; these are building up, or constructive, 

 processes and require a supply of energy from the sun or the living 

 protoplasm to bring them about. This food is then united with 

 oxygen, thereby releasing this sun-given energy. It is this energy 

 which keeps the plant alive and permits it to grow. This last 



