38 ELEMENTARY STUDIES IN BOTANY 



therefore, may be thought of as the first stage of food, and 

 proteins as the last stage. 



The constitution of proteins is not known, so that their 

 manufacture is not understood. It is known that neither 

 light nor chlorophyll is required, for the process goes on in 

 living cells removed from light, and in plants containing no 

 chlorophyll. It is known, however, that carbohydrates are 

 used, and that to the carbon, hydrogen, and oxygen supplied 

 by them, the elements nitrogen, sulphur, and often phos- 

 phorus are added. It is important to know the sources of 

 these new elements that enter into food manufacture. They 

 are not used by the plant as free elements, but are obtained 

 from their combinations in what are called salts. For ex- 

 ample, salts containing these elements occur in all soils upon 

 which plants can grow, and these same salts are dissolved 

 in the water in which AlgaB grow. In land plants, they enter 

 through the roots, while in Algae they enter wherever the 

 plant is exposed to water. 



30. Assimilation. While the processes of food-manu- 

 facture are being considered, it will be helpful to define the 

 use of food. There is an intermediate process called digestion, 

 which simply means the conversion of foods into transfer 

 forms, usually soluble forms. For example, digestion trans- 

 forms insoluble starch into soluble sugar. It is evident, 

 furthermore, that only those foods need to be digested which 

 are not in transfer form. 



The process by which foods are used in the manufacture 

 of protoplasm is called assimilation. Protoplasm is the 

 living body and it uses food to construct more protoplasm. 



31. Respiration. Everything about the plant is a pro- 

 duct of protoplasm, and in doing the great variety of work 

 that goes on in a living body the protoplasm " breaks down," 

 using itself up continually in the manufacture of products. 

 Of course this explains why it must be assimilating all the 

 time, so that its body may be continually built up. This 



