176 TEXTBOOK OF BOTANY 



before summary statements can be made. They are soluble in water and 

 may pass from cell to cell; hence they may be made in one part of the 

 plant, such as leaves, and influence the development of cells in another 

 part of the plant. General effects resembling those due to natural hor- 

 mones may be brought about by closely related compounds sold by 

 chemical supply companies. 



Such substances as enzymes, vitamins, and hormones are the modern 

 substitutes for the earlier assumptions of special vital forces and en- 

 telechies. Not only are development and health dependent upon the 

 presence of these substances, but certain differences in species are de- 

 pendent upon their systems of enzymes, vitamins, and hormones. 



Assimilation. It is often convenient to have one word by which we 

 may refer to the transformation of food into the substances of which cells 

 are composed. The term assimilation has been used with that intent in 

 biological sciences for a long time. But it has also been given different 

 interpretations as the knowledge of plant and animal nutrition advanced. 

 One therefore finds it used today as it might have been used a century 

 ago, and also as it probably should be used on the basis of present 

 knowledge. A number of other expressions likewise became associated 

 with the concept of assimilation at different historical periods, and they 

 have also survived to the present time. Other biological terms, such as 

 food, respiration, osmosis, adaptation, natural selection, and many others 

 have a similar history. All of them, as well as the associated expressions, 

 need to be critically examined by each succeeding generation of students. 



When protoplasm was first recognized it seemed so all-important, 

 especially in the animal body, that the term assimilation was, and still 

 is, sometimes restricted to the transformation of food into protoplasm 

 alone. This is a definite use of the term; but to adopt it we must seek 

 another term to refer to the transformations of food into cell walls and 

 other essential constituents of cells. It seems most convenient to use one 

 term to include all of these processes in both plant and animal bodies. 



The greatest confusion arose before the discover\^ that the food of 

 green plants and of man is essentially the same, regardless of the manner 

 in which they obtain it. Assimilation is really one phase of growth that 

 is common to all plants and animals. It is quite different from the forma- 

 tion of sugar from CO2 and H-O or of amino acids and proteins from 

 sugar and certain inorganic salts. That is, food manufacture and the 

 subsequent use of food in assimilation are two different groups of bio- 

 logical processes. 



