102 TEXTBOOK OF PLANT PHYSIOLOGY 



fectly dormant seeds showing no signs of life, the water content is 

 but 10 to 15 per cent. Seeds give an excellent illustration of how 

 much the vital processes in plants depend on a definite degree of 

 saturation. Perfectly viable seeds containing only hygroscopic 

 water may remain unaltered during many months, and even years, 

 without showing any signs of life. As soon as they are soaked in 

 water, they at once begin to germinate. If seeds obtain less water 

 than is required for their complete saturation, but more than they 

 usually contain when air dried, for instance 20 to 25 per cent, 

 respiration, the elimination of carbon dioxide and absorption of 

 oxygen is initiated. This as well as a number of other processes 

 may eventually lead to the loss of power of germination. Under 

 these conditions, no growth is secured. This incomplete awaken- 

 ing of the seeds by partial moistening is of great importance. It 

 shows that various vital processes require different degrees of 

 saturation with water. The highest amount is necessary for 

 growth and other processes connected with constructive activities, 

 the lowest, for respiration and other processes of degradation. 



One of the reasons why water is so essential for the vital func- 

 tions in a plant is the fact that many chemical reactions may 

 be effected only in a water solution. Protoplasm itself, the prin- 

 cipal substrate of all vital processes, must be saturated with water 

 in order to be active. Protoplasm is often defined as a semi- 

 liquid substance of mucilaginous character. Such substances 

 have been termed colloids, which means gluelike, or substances in 

 the colloidal condition. It has been, established in recent years 

 that the same substances may be obtained in crystalloidal as well 

 as in colloidal state. 



34. General Conception of the Colloidal State. Fundamental 

 Properties of Colloids. Phenomenon of Swelling. — The colloidal 

 condition of a substance, in terms of physical chemistry, is a state of 

 very high dispersion, while the medium in which the minutely 

 divided particles are suspended is called the dispersion medium. 



Three types of dispersed systems may be distinguished: (1) 

 coarsely dispersed systems with particles of the dispersed phase not 

 less than 0.1/x (micron = thousandth of a millimeter); (2) col- 

 loidal systems in which the particles of the dispersed phase range 

 from 0.1m to 0.001/z; and (3) molecular, or ionic, dispersed systems 

 in which the division in the dispersed phase is so great that it is 

 represented by separate molecules or ions that have lost all connec- 



