PHYSIOLOGY OF PLANTS 



I. NATURE AND RELATIONS OF AN ORGANISM 



1. The Constitution of Living Matter. The properties of any 

 mass must depend upon those of its constituents. Living mat- 

 ter is composed chiefly of carbon, hydrogen, oxygen, and nitro- 

 gen, while sulphur and phosphorus are essential constituents in 

 smaller proportions. A chemical and physical examination of 

 these substances shows that they exhibit the most widely dissimilar 

 characteristics. Thus carbon exhibits a greater degree of atomic 

 cohesion than any other known element, and may be liquefied 

 and volatilized only at extremely high temperatures, while oxygen, 

 hydrogen and nitrogen are gaseous at ordinary? temperatures and 

 undergo liquefaction and solidification only at very low tempera- 

 tures. Oxygen displays the greatest range of chemical affinity 

 and intensity. Hydrogen and carbon have a low chemical inten- 

 sity and a very narrow range of chemical affinity, while nitrogen 

 is inert. Carbon, sulphur and phosphorus undergo allotropic 

 modifications, and some of the oxides present in living matter are 

 isomeric. The union of elements of such varying properties gives 

 protoplasm a molecular mobility and chemical activity that en- 

 ables it to undergo the changes in the arrangements of its parts 

 constituting development, with great readiness. Furthermore any 

 incident force falling upon substances of such great dissimilarity in 

 chemical activity must give rise to many kinds of transformations of 

 energy constituting the functions, each capable of infinite modifica- 

 tion ; as if a number- of bars of different sizes and of different 

 kinds of metal were suspended freely and all should be struck by 



