136 PLANT PHYSIOLOGY 



dry seeds contain only hygroscopic water in equilibrium with 

 the ordinary atmosphere, when the moisture content is usually 

 about 10 to 12 per cent of the dry weight. Such seeds may 

 remain during many years without any signs of germination and 

 produce carbon dioxide in very low amounts. But as soon as 

 they are soaked in water, they begin to germinate, provided that 

 other necessary conditions are adequate, such as the temperature 

 and the aeration. If the seeds are given not the 50 to 60 per 

 cent of water necessary for their complete saturation but only 

 about half of this quantity, the process of respiration, as well as 

 the beginning of hydrolytic decomposition of the reserve sub- 

 stances, will be perceptible, but growth will not take place. 

 Such an incomplete excitation of the vital activity from partial 

 moistening of the seeds is of great importance theoretically. It 

 proves that various vital processes require a different degree of 

 saturation with water, the greatest amount being necessary for 

 growth. In general, much water is required for all functions 

 connected with the formation of new masses of Uving substances, 

 and considerably less for respiration and for digestion of the food 

 reserves of the seed. 



Walter (1931) designated the degree of saturation of the proto- 

 plasm with water by the term "hydrature.'' The amount of 

 water is determined not so much by the total water content of 

 the protoplasm or of the cell, expressed in percentage of the dry 

 weight, as by the force with which the cell absorbs water from the 

 surrounding medium. Walter measured the degree of saturation 

 by the vapor pressure at the surface of the cell or other bodies 

 impregnated with water. When completely saturated with 

 water, the cell will be in equilibrium with an atmosphere satu- 

 rated with vapor, and its hydrature may be expressed as 100 per 

 cent. When placed in an unsaturated atmosphere, a moist body 

 will lose water, but simultaneously its water-holding capacity 

 will begin to increase. If the vapor pressure of the surrounding 

 atmosphere is 95 per cent, the moist object may cease to lose 

 water only after an increase of its water-retaining capacity up to 

 a magnitude that corresponds to the vapor pressure of a solution 

 with an osmotic pressure of 55.8 atmospheres, at a tempera- 

 ture of 20°C. It is well known that an increase in the concen- 

 tration of a solution is accompanied by a corresponding increase 

 in its osmotic pressure and a decrease in the vapor pressure at 



