THE RELATION OF PLANTS TO WATEK 145 



following figures, derived from the experiments of Sachs, will 

 show. Sachs states that a particle of water may travel as much 

 as 100 centimeters (40 inches) per hour in the vessels of woody 

 plants. He found that in one species of willow the water 

 moved 85 centimeters per hour, in corn plants from 30 to 42 

 centimeters, in the sunflower 70 centimeters, and in the grape 

 98 centimeters. With these facts in mind we may now turn 

 to the forces available in the plant for transporting these great 

 volumes of water, at the rates indicated, from the roots to the 

 crown of woody and herbaceous plants. 



In herbaceous plants it is conceivable that capillarity, or the 

 rise of water in the ducts clue to root pressure, might accomplish 

 the work involved in water ascent. It has been found, however, 

 that capillarity is not effective for this purpose in tubes as large 

 as the ducts of our common plants, and that root pressure, while 

 important in the spring, before transpiration begins, is practically 

 in abeyance during the periods of vigorous transpiration. In tall 

 trees these same forces would be much less adequate than in the 

 smaller herbaceous plants. As indicated above, no theory has 

 ever been advanced to explain satisfactorily all the aspects of 

 water ascent, but there are certain known physical factors which 

 should be mentioned as furnishing a partial explanation for the 

 phenomena. These factors, taken together, constitute what is 

 often called the cohesion theory for water ascent. The cohesion 

 theory is based upon the cohesive power of small columns of 

 water, which is known to be very great when subjected to a 

 straight pull and is variously estimated at from 10 to 150 atmos- 

 pheres. If this cohesion of a water column applies to the water 

 in the condition in which it exists in the ducts and stems of 

 plants, a scientific explanation of water ascent is conceivable on 

 this basis. The entire column of water in a tree trunk would 

 then act like a rope or chain and could be drawn upward as a 

 whole if a sufficient lifting force were applied at the top. This 

 lifting force is believed by the advocates of the cohesion theory 

 to exist in the osmotic suction of the mesophyll cells bordering 

 on the veins of a leaf. As these cells evaporate water into the 

 intercellular spaces of the leaf the cell sap becomes concentrated 



