I I 2 THE PLANT WORLD. 



greater attraction for each other than tliey have for water. In a 

 substance that swehs only to a hmited extent it seems necessary to 

 postulate a difference of attractive force among the molecules in 

 different directions ; for otherwise liquid would continue to be 

 injected, though with gradually decreasing force, until the whole 

 substance went into a condition resembling solution. We may 

 sav, therefore, that water will continue to be absorbed by such a 

 substance until the attractive forces of the molecules in the 

 substance in direction of their strongest action equilibrate with the 

 constantly diminishing force of penetration of the liquid. It must 

 of course be assumed that the attractive forces among the mole- 

 cules of the substance in their strongest direction are stronger than 

 those of the fluid for the same molecules. 



From this it follows that the first particles of the fluid pene- 

 trate and are held with greatest force. Since the molecular forces 

 in all substances act only through an exceedingly short distance, 

 and the meniscus of the capillary in-flowing streamlets becomes 

 rapidly less concave as the streamlets grow larger, the force of 

 imbibition decreases very rapidly. As quoted by Pfeft'er, Reinke 

 found in a frond of Laminaria that while it required but sixteen 

 atmospheres pressure to squeeze out water when the content was 

 63 per cent., it required two hundred atmospheres to cause the 

 same result when the content was reduced to 48 per cent. This 

 point becomes of very great importance in the theory of freezing, 

 as we shall see later. The force of imbition for the first molecules 

 absorbed is often enormously great. Rodewald found that this 

 for starch was equal to about 2,523 atmospheres. Hales found 

 that swelling peas could lift a weight of 83.5 K. or about 185 

 pounds. An experiment instituted by Sachs showed the great 

 strength of this force in an impressive manner. He found that 

 if asphalt lack was spread upon a very dry bladder-membrane or 

 bibulous paper, and then well dried, it would adhere to these 

 substances with enormous force. An attempt to separate the two 

 usually resulted in tearing oft" thin layers of the membrane or 

 paper. If, however, the paper was brought in contact with water, 

 so that imbibition commenced, the lack would soon be separated 



