266 PLANT PHYSIOLOGY 



Askenasy (1896), in an experiment the importance of which 

 ranks it among the classical experiments of plant physiology, 

 showed that this was a purely physical process by replacing the 

 twig with a dead animal membrane or even by porous gypsum. 

 As the water evaporated from the gypsum, mercury was drawn 

 up from below, in the same manner as in Bohm's experiment, to a 

 height of 80-85 cm., which is also seen to be greater than the height 

 sustained by the atmosphere. Such experiments not only give 

 an idea of the force of evaporation but of the cohesion of the 

 water as well. 



The water particles cohere extremely tightly to each other but 

 they also adhere to the walls of the cells in which the water columns 

 are being pulled upwards. This adhesion is so great that when a 

 tree is actively transpiring the walls of the vessels and tracheids 

 are drawn inward slightly as when one draws soda water through 

 a straw. To prevent this, the walls of these cells have spiral and 

 annular thickenings; but that some compression results, neverthe- 

 less, may be seen by measuring with a sensitive instrument such 

 as the dendograph designed by MacDougal. This apparatus shows 

 that on bright days when transpiration is active, the diameter of 

 a tree is slightly less than at night when transpiration is much less. 



The energy for the transpiration pull is, of course, supplied by 

 the sun. It has been previously shown that only a small amount 

 of the sun's energy is used in photosynthesis, the major part of 

 what is absorbed being used in heating the plant and in evaporating 

 the water. Ganong has calculated that, in a big tree, the work 

 done in one day in lifting the water from the roots to the leaves is 

 equivalent to that done by a man in carrying 500 ten-quart pails 

 full of water up a ten-foot stairs. 



Objections to the Theory. — While the cohesion theory is the 

 best and most satisfactory that we have, there are certain objec- 

 tions which have been outstanding. These have largely centered 

 around (1) the gas bubbles in the wood and (2) the cross walls 

 at the ends of the cells. 



Will not the gas bubbles break the columns of water? It is a 

 known fact that as the plant transpires rapidly and the negative 

 pressure develops in the stem, the dissolved gases which are in the 

 sap expand and come out of solution just as the bubbles appear 

 in a bottle of soda water when the cap is removed. Why such 

 bubbles do not break the water columns and so render the co- 



