Chapter II 

 STRUCTURE OF WATER 



Introduction : — Most cells originate in a highly aqueous medium ; they 

 use water in growth and other vital processes ; in most instances, they con- 

 sist largely of water throughout their life span. Logically there should be 

 an intimate relation between the functions of cells and the properties of 

 water and dilute aqueous solutions. A study of plant water relations im- 

 presses one that this is true. 



Water enters into the composition of carbohydrates, proteins, and other 

 compounds making up the walls and protoplasts of plant cells ; it is com- 

 bined with various colloidal constituents of cells as water of hydration ; it is 

 involved in many metabolic processes in the plant ; and it exists as a liquid 

 in the solutions occupying the vacuoles of living cells and the lumina of 

 conducting elements. Due to the many forces tending to reduce the activity 

 of water molecules, it may be difficult to distinguish between the various 

 states in which water exists within the plant. 



Combining the elements hydrogen and oxygen, water in its simplest 

 form corresponds to the formula H2O. Many more complicated forms 

 have been proposed to explain its various properties. Most of these in- 

 volve association or polymerization. 



Water boils at 100° C. and freezes at 0° C. when under one atmosphere 

 pressure; it is liquid throughout the temperature range at which plants 

 thrive. Many plants can survive freezing temperatures ; certain spores can 

 stand boiling; photosynthesis and transpiration may take place through a 

 range from near freezing to the thermal death temperature around 40° to 

 50° C. Many plant functions, however, find their optimum within the 

 range from approximately 20° to 30° C. The occurrence over a large por- 

 tion of the earth's surface of temperatures within these narrow limits de- 

 pends, among other things, upon the large latent heats of vaporization and 

 freezing of water, and upon its heat capacity and conductivity. Hender- 

 son (1924), discussing the fitness of the environment, points out the many 

 unusual properties of water, most of which apparently contribute in some 

 way to the well-being of plants. It is interesting to note the exceedingly 

 narrow range of temperature and moisture within which our important 

 economic plants thrive, and to point out the role played by water in main- 

 taining these conditions. 



Structure: — Water has long been recognized as a compound of unusual 

 character. Tables 1 and 2 list some of the physical properties of common 

 elements and liquids. Water stands out, having a very high heat of vapori- 

 zation and a fairly high heat of fusion. (For further data of this type, see 

 Bernal and Fowler, 1933, Table III). Of the compounds listed, water 

 has the greatest surface tension, internal pressure, and dielectric constant. 

 These latter properties indicate the interatomic forces present in the mole- 

 cule. 



Formed from hydrogen which boils at — 253° C. under atmospheric 

 pressure, and oxygen which boils at — 180° C, water boils at 100° C. If 

 normal in its properties water should boil at a very low temperature vari- 

 ously estimated at from — 65° C. to — 100° C. Its freezing point should 



