114 



THE PLANT WORLD. 



When a substance passes from the hquid to the soHcl state 

 through change in temperature or pressure, in many cases it does 

 so by the formation of crystals. The size of the crystals depends 

 mainly upon the accessibility of the liquid and the rate of fall in 

 temperature, or rise in pressure, being smaller when this is rapid 

 and when the fluid is more inaccessible, and vice versa. 



Fig. i8. Diagram illustrating the water-system concerned in the form- 

 ation of ice crystals in intercellular spaces, a. Intercellular space ; b, 

 prismatic ice crystal ; c, superficial water film ; d, cell wall ; e, protoplasm ; 

 f, vacuole containing cell sap. 



The formation of a crystal is accompanied by the exhibition of 

 considerable force on the part of the molecules on entering into it. 

 To prevent more molecules of the liquid from being applied and 

 added to the crystal an enormous force is often required. This 

 is nowhere better shown than in the expansive force of newly 

 forming ice which can even rend rocks asunder, break iron bands, 

 etc., when properly applied. This, however, is molecular force 

 pure and simple, and can act oidy at an infinitesimal distance from 

 the forming crystal. Material can be drawn to a forming crystal 



