PLANT FOOD 69 



ing a higher percentage of carbon. They are more con- 

 centrated than starch. When burned, one pound of fat 

 produces about two and one-fourth times as much heat 

 as is produced by a pound of starch or sugar. One pound 

 of fat is equal to about two and one-fourth pounds of 

 starch or sugar as a food for plants or animals. 



68. Protein is a term used to cover a large number of 

 different compound?. They all contain nitrogen. They 

 may be more properly called nitrogenous compounds. 

 They are chiefly composed of carbon, hydrogen, oxygen 

 and nitrogen. Most of them contain phosphorus, sulfur 

 and other elements. 



69. Plants the Only Source of These Foods. None of 

 these compounds can be made in any way except by plants. 

 A chemist cannot get the carbon, hydrogen and oxygen 

 to unite, so as to form starch. Therefore, all animal life 

 depends on plant life. 



70. Stored Food. The food that is stored in a plant is 

 nearly always in insoluble compounds, such as starch, 

 oils, insoluble forms of protein. In this form it is less 

 easily damaged. 



71. Periods in the Life of a Plant. There are two rather 

 distinct periods in the life of a plant: (1) The period of 

 growth and formation of food; (2) the period of repro- 

 duction. 



These stages are very marked in some plants, as 

 turnips, that grow and store up food during one season, 

 and that transfer the food to the seeds during the next 

 season, and then die. 



With trees the stages are less marked. It is a well- 

 known fact among fruit-growers that a good growth for 



