INTRODUCTION 



THE history of biological science shows that the conceptions 

 which men have held concerning the nature of plant and animal 

 growth have undergone a series of revolutionary changes as the 

 technique of, and facilities for, scientific study have developed 

 and improved. For a long time, it was thought that life processes 

 were essentially different in character than those which take place 

 in inanimate matter, and that the physical sciences had nothing 

 to do with living changes. Then, too, earlier students had only 

 vague notions of the actual structure of a living organism. Begin- 

 ning with the earliest idea that a plant or an animal exists as a 

 unit organism, to be studied as such, biological science progressed, 

 first to the recognition and study of the individual organs which 

 are contained within the organism; then to the tissues which make 

 up these organs; then (with the coming into use of the microscope 

 as an aid to these investigations) to the cells of which the tissues 

 are composed; then to the protoplasm which constitutes the 

 cell contents; and finally to the doctrine of organic evolution as the 

 explanation of the genealogy of plants and animals, and the study 

 of the relation of the principles of the physical sciences to the 

 evolutionary process. The ultimate material into which organisms 

 are resolved by this process of biological analysis is the cell proto- 

 plasm. But protoplasm is itself made up of a complex system of 

 definite chemical compounds, which react and interact according 

 to the laws of physical science. Hence, any study of the chem- 

 istry of plant growth is essentially a study of the chemical and 

 physical changes which take place in the cell protoplasm. 



Protoplasm differs from non-living matter hi three respects. 

 These are (1) its chemical composition; (2) its power of waste 

 and repair and of growth; and (3) its reproductive power. From 

 the standpoint of chemical composition, protoplasm is the most 

 complex material in the universe. It not only contains a greater 

 variety of chemical elements, united into molecules of enormous 



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