TRANSLOCATION OF SUBSTANCES IN THE PLANT 329 



the trunk and cut shoots. As soon as the leaves are completely 

 unfolded, the force of transpiration will be so much greater than 

 root pressure that cut roots will even absorb water. When 

 larger roots are cut in summer, their surface does not exude 

 water but absorbs it. 



In the trunk of a tree, the suction tension of the leaves is 

 transmitted downward to a considerable distance. It is known, 

 how^ever, that a suction pump is not able to hft water to a greater 

 height than about 30 ft. At this height, the weight of the water 

 column is equal to the pressure of 1 atmosphere. Pumping a 

 perfect vacuum will not induce water to rise above this level, 

 because the rise of water is due to the pressure of the atmosphere. 



The cause of such a sharp difference between the lifting of 

 water by a pump and the sucking of water in the vessels of the 

 plant is explained by the fact that in plant vessels there are no 

 air bubbles, which are always present on the surface of the walls 

 of pumps. These air bubbles expand under tension, and the 

 cohesion of the w^ater column is broken. 



The force of attraction between the water molecules is called 

 "cohesion." It is the same force that holds an iron bar together, 

 making it possible to lift it by one end. The force of cohesion in 

 solid bodies may be determined in a comparatively simple manner 

 because of the low mobility of their molecules and their constant 

 external form. When the tensile strength of a bar of metal is 

 tested, the strain at which it breaks is considered as being equal 

 to the force of cohesion. Thus a steel bar, for instance, breaks 

 at a strain of about 80 kg. per square millimeter, which approxi- 

 mately corresponds to the tension of 8,000 atmospheres. 



Since Uquids have molecules in a more mobile state, tests of 

 the magnitude of their cohesive force are subject to greater diffi- 

 culties and errors. The determination has to be conducted in a 

 tube with two very closely fitting pistons. It has been proved 

 by means of proper equipment that the cohesive force of water 

 molecules and their adhesion to the surface of the pistons are very 

 high. Only by applying to the pistons a pulling force of many 

 atmospheres can the water layer between the pistons be ruptured. 

 Still it is difficult to establish whether the water column itself 

 has been ruptured or whether it has only been torn away from 

 the surface of the piston or of the tube. The latter seems more 

 probable, since on the surface of glass or metal there is always a 



