[Chap. XI A BIT OF USEFUL CHEMISTRY 97 



ions. Chemical unions may also occur without a complete transfer of 

 electrons from one atomic system to another. When two atomic systems 

 approach each other, some of the outermost planetary electrons may be 

 mutually held (shared) by both systems. Electrostatic forces hold the 

 two atoms together as one molecule. Thousands of atoms may thus be 

 held together in the largest molecules of plants and animals. 



This incessant activity of electrically charged units within the atom 

 makes its capacity for union with other atoms more easily understood. 

 All of this internal activity within a molecule, say of water, may well 

 cause us to wonder at its stability and uniformity of behavior. When 

 we further attempt to picture the complex molecules and unceasing 

 activities within every unit of a living cell, the regularity of development 

 and the stability of living organisms composed of millions of cells are 

 still more remarkable. 



Our imagination is further taxed by fossil records which show that 

 certain species of both plants and animals are so stable that they have 



Fig. .50. Navicula lijra, a diatom present in marine deposits formed 25 to 30 

 million years ago. This species still lives today on sea coasts throughout the world. 

 Photo by Spencer Lens Co. 



remained apparently unchanged generation after generation through 

 millions of years (Fig. 50). The stability of the cell, of the whole or- 

 ganism, and of the species is in the last analysis dependent upon the 

 stability of physical-chemical units and upon the orderliness with which 

 cycles of processes are repeated untold numbers of times. Cellular ac- 

 tivity is far easier to comprehend than cellular stability; but this very 

 stability is evidence of the inherent orderliness of natural processes. 



Chemical processes in plants. An astounding number of different sub- 

 stances is formed in plants. Yet the formations and transformations of 

 these substances involve only a few fundamental chemical processes. 



