60 PLANT GROWTH AND NUTRITION 



Food in the Cotyledons. - The problem now before us is to find 

 out how the embryo of the bean is adapted to grow into an adult 

 plant. Up to this stage of its existence it has had the advantage 

 of food and protection from the parent plant. Now it must begin 

 the battle of life alone. We shall find in all our work with plants 

 and animals that the problem of food supply is always the most 

 important problem to be solved by the growing organism. Let 

 us see if the embryo is able to get a start in life (which many 

 animals get in the egg) from food provided for it within its own 

 body. 



Organic Nutrients. - - Organic foods (those which come from 

 living sources) are made up of two kinds of substances, the nutri- 

 ents or food substances and wastes or refuse. An egg, for example, 

 contains the white and the yolk, composed of nutrients, and the 

 shell, which is waste. The organic nutrients are classed in three 

 groups. 



Carbohydrates, foods which contain carbon, hydrogen, and 

 oxygen in a certain fixed proportion (C 6 Hi O 5 is an example). 

 They are the simplest of these very complex chemical compounds 



we call organic nutrients. Starch and sugar 

 are common examples of carbohydrates. 



Fats and Oils. - These foods are also com- 

 posed of carbon, hydrogen, and oxygen in a 

 proportion which enables them to unite 

 readily with oxygen. 



Proteins. - -A third group of organic foods, 

 Starch grains in the cells proteins, are the most complex of all in 



of a potato tuber. ... . , 



their composition, and have, besides carbon, 



oxygen, and hydrogen, the element nitrogen and minute quantities 

 of other elements. 



Test for Starch. - - If we boil water with a piece of laundry starch 

 in a test tube, then cool it and add to the mixture two or three 

 drops of iodine solution, 1 we find that the mixture in the test tube 



1 Iodine solution is made by simply adding a few crystals of the element iodine 

 to 95 per cent alcohol ; or, better, take by weight 1 gram of iodine crystals, f gram 

 of iodide of potassium, and dilute to a dark brown color in weak alcohol (35 per 

 cent) or distilled water. 



