How Plants Draw in Various Materials 



179 



condition of balance in a saturated membrane explains how it 

 is that water can pass so readilj^ through it ; for the last films 

 absorbed, those farthest from the micellae, are held so verj'- 

 lightly that only a slight force is required to draw them from the 

 membrane. \^^hat the nature of the 

 force may be which withdraws the 

 water from the inner face of the mem- 

 brane in osmosis we shall consider in 

 a moment. 



Diffusion, imbibition, osmosis it- 

 self are typical examples of molec- 

 ular forces, those operating between 

 individual molecules, in contrast with 

 the more familiar molar forces which 

 act upon masses. There is also one 

 other molecular force of some im- 

 portance in the plant, — viz., capil- 

 larity, which we must now briefly 

 notice. Capillarity is the well- 

 known force by which water is raised 

 in small tubes, — or any small pas- 

 sages no matter how irregular, — and 

 the higher the finer the tubes, as our 

 diagram illustrates (figure 60). It 

 is the power by which a towel dries 

 water from the skin, a blotter takes up ink, a wick raises 

 oil, or any porous substance soaks up liquids. It is only with 

 difficult}^, and under suasion from my critic, that I forbear to 

 explain this interesting process in detail to the reader; and I 

 must regretfully confine my exposition to the following brief 

 synopsis. The capillary rise of water is due to forces residing 

 within the water itself. Because the attractions mutually exerted 

 between the molecules inside of the liquid are not balanced at 

 the surface by equivalent attractions towards the outside (fig- 



FiG. 60. — A diagram to illustrate the 

 rise and depression of liquids in 

 capillary tubes, drawn to approxi- 

 mately true scale. The liquid on 

 the left is mercury, and on the right 

 is water. 



