MEASUREMENT OF THE INTERNAL FORCES 

 OF SEEDS. 



l'.\ diAKr.ios A. Siui.i,. 



IT IS a well-known fact that when dry organic matter of 

 any kind, as dry seeds, is brought into contact with water 

 it imbibes moisture with great rapidity and swells with con- 

 siderable energy. The water enters the swelling body in re- 

 sponse to powerful internal forces which owe their existence 

 and also their magnitude to the fact that the organic matter 

 is composed of extremely minute particles which exhibit sur- 

 face force. These forces, sometimes referred to as imbibition 

 force, capillarity, surface force, etc., will be designated here 

 simply as internal forces, since we have to deal with the re- 

 sultant of a number of forces as expressed in the attraction of 

 seed substance for water. 



Furthermore, the universal presence of hygroscopic mois- 

 ture in such organic matter is well known, and the fact that 

 this moisture varies in quantity from day to day is a common 

 observation. At all times the amount of water present in an 

 air-dry seed, for instance, is determined by the balance be- 

 tween external forces which tend to remove water from the 

 seed, and internal forces which resist this removal and which 

 immediately cause moisture intake whenever they happen to 

 exceed the external forces. In other words, moisture flows 

 into or out of the seed as one set of forces or the other is the 

 greater. 



It must be noted, also, that the relation of these forces to 

 water itself is such that as the water flows back and forth in 

 response to given physical conditions, the greater force is 

 always lessened by the movement of water, while the lesser 

 force is simultaneously increased by that same movement of 

 water, for instance, when the evaporating power of air is 

 higher than the internal water-holding forces of the seed, the 

 seed dries out. The moisture leaving the seed decreases the 

 evaporating power of the air by increasing its vapor pressure, 

 while the internal forces of the seed increase with surprising 

 rapidity as the seed dries out, due to the thinning of the in- 

 ternal surface films. Again, if the surface force of a soil were 



(65j 

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