BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



109 



ments were made through a series of temperatures ranging from 

 25° to 80°C. 



Renner 12 and Ursprung 13 working independently, have re- 

 cently used the annulus cells of the spore cases of fern as a 

 means of measuring the cohesion of water. Both investigators 

 find that the cell contents of many of the spore cases may be 

 brought into equilibrium (through the vapor phase) with solu- 

 tions having an osmotic pressure of 300 atmospheres before the 

 limit of cohesion is reached and air bubbles appear in the 

 cells. All of these experiments appear to support the 

 conclusion that the cohesion of water is amply 

 sufficient to withstand the stresses in the stem of 

 a tree. 



The cohesion of water may be easily demon- 

 strated, as Dixon has shown, by means of a 

 J-shaped sealed tube (fig. 6). A quantity of 

 water more than sufficient to fill the lower limb 

 is introduced into the tube, which is then par- 

 tially exhausted and sealed off. It is unneces- 

 sary to reduce the air pressure below 2 cm. of mercury; in 

 other words, the water may contain very appreciable quan- 

 tities of air. If the tube is now first inclined so that the 

 water fills the longer limb completely, which may be 

 over a meter in length, and is then carefully raised 

 to the position shown in figure 6, the water column 

 remains hanging from the top of the tube. On de- 

 stroying the cohesion of the column at some point 

 by a sharp blow, or preferably by heating a stout 

 platinum wire previously sealed through the wall of 

 the tube for this purpose, the water column sudden- 

 ly falls to the level determined by the gas pressure in the bulb. 



Cohesion may also be readily demonstrated by a Bourdon 

 spring connected with a bulb through a flexible capillary tube. 



Fig. 6. Sealed 

 tube for illus- 

 t rating the 

 cohesion of 

 water. The 

 water column 

 is shown hung 

 from the top 

 of the long 

 tube. 



r- 



12 Renner, 0. Theoretische und Experimentalles zur Kohasionstheorie der 

 Wasserbewegung. Jahrb. Wiss. Bot., 56: 617-667. 1915. 



13 Ursprung, A. Ueber die Kohasion des Wassers in Farnannulus. Ber. 

 Deutsch. Bot. Gesells., 33: 153-162. 1915. 



