How Plants Draw in Various Materials 181 



climbs up a bit on the wall, as our figure 61, c well illustrates, mak- 

 ing the surface concave to a degree that is greater the smaller the 

 tube. Hence the greater surface tension of the flat surface outside 

 pushes the mobile water up against the lesser pressure of the con- 

 cave surface inside, forcing it to rise against gravitation until equi- 

 librium is established, which will occur at a higher point the smaller 

 the tube. And the reverse process occurs with liquids which 

 will not adhere to glass or wood, e. g., mercury, or with walls 

 of such composition that water will not adhere thereto, as in 

 some air passages of plants; for in this case the surface in the 

 tube is convex, and presses the water down against the flat sur- 

 faces outside, so that the liquid stands below the outside level in 

 the tube (figure 60, on the left), or, if the tube is not deeply im- 

 mersed, will not enter at all. 



Such is capillarity, deriving its energy from internal molecular 

 tensions given release by peculiarities of external conditions, and, 

 like all molecular forces, strictly limited in amount and without 

 possibility of continuous action. Capillarity plays in the plant 

 some minor part in the ascent of sap, in prevention of the entrance 

 of water into some air passages, and in other processes later to be 

 noted. Moreover, some physicists see in imbibition nothing but 

 a refined capillarity, although as I think, the phenomena of im- 

 bibition of water vapor, presently to be noted under hygrosco- 

 picity, is hardly consonant with this explanation. Still another 

 possible connection of a refined capillarity with osmotic absorp- 

 tion will be noticed in a moment. 



We have now reached the place where the reader who may 

 have used my permission to skip for a little must resume his 

 grasp on this narrative if he is to understand the essentials of 

 osmotic phenomena. 



In watching the ascent of a liquid in an osmoscope, like that 

 of figure 56, one sooner or later comes to wonder what would 

 happen in case an insuperable barrier, e. g., a tight stopper, were 

 interposed against the further rise of the liquid. The matter is 



