CHAPTER XI 

 A BIT OF USEFUL CHEMISTRY 



Many gross features and properties of plants may be perceived by means 

 of our unaided senses. With the help of a microscope, tissues and cells 

 can be distinguished, and with the highest powers of this instrument 

 many minute and important structures within the cells can be studied. 

 These visible units of a plant are composed of invisible units, which 

 may be investigated and mentally visualized by the methods devised by 

 chemists and physicists for studying the composition and transforma- 

 tions of all matter. To understand the visible structures of a plant and 

 the processes by which these structures are built and broken down, cer- 

 tain definite ideas about the invisible units of matter are essential. Con- 

 sequently, it may be helpful to consider briefly here a few invisible units 

 of matter and some of the usual chemical processes by which these units 

 may be combined or changed. These general ideas and principles will be 

 amplified and applied in subsequent discussions of phvsiological proc- 

 esses of plants. 



Molecules and atoms. Like all other objects of our environment, plants 

 are composed of molecules of definite chemical composition. The proper- 

 ties of the microscopically visible parts of a plant — the protoplasmic 

 structures and cell walls — are determined in part by the kinds of mole- 

 cules of which they are composed, and in part by the arrangement and 

 organization of these molecules into aggregates. Each molecule of a par- 

 ticular compound such as water, sugar, or cellulose is made of still 

 smaller units of matter that are definite in kind and in structural ar- 

 rangement. The characteristic properties and reactions of molecules de- 

 pend upon the presence and arrangement of these smaller units, the 

 atoms and ions. 



Water is a familiar compound, and in its simplest fomi its molecule 



consists of 2 atoms of hydrogen and 1 atom of oxygen — briefly designated 



+ 

 as H2O. Other designations are H — O — H and H — OH; the first of 



these indicates that both atoms of hydrogen are directly combined with 



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