234 THE MOVEMENT OF WATER THROUGH THE PLANT 



tubes of small diameter the existence of attractive forces between water 

 molecules can often be demonstrated. If the water at the top of such a tube 

 be subjected to a "pull" the resulting stress will be transmitted all along 

 the column of water, due to the mutual attraction between the molecules. 

 The water conducting system of plants constitutes just such a sj'stcm, enclos- 

 ing continuous thread-like columns of water which extend from the top to the 

 bottom of the plant. A stress applied at any point to this system will be 

 propagated to all its parts. The application of such a stress stretches the 

 water into taut threads, throwing it into a condition of tension, which is the 

 equivalent of "negative pressure." 



According to this theory forces develop in the upper parts of plants, and 

 especially in the leaves, which cause the rise of water through the plant. 

 As evaporation proceeds from the walls of the mesophyll cells into the inter- 

 cellular spaces the diffusion pressure deficit of the mesophyll cell walls in- 

 creases. Water therefore moves into the walls from the adjacent protoplasm, 

 this resulting in turn in the movement of water from the cell sap into the 

 protoplasmic layer. The resulting increase in the diffusion pressure deficit 

 of the cell sap is in turn propagated to the protoplasm and the cell sap in 

 all parts of the cell. The osmotic pressures of the leaf cells are generally 

 high enough that diffusion pressure deficits of sufficient magnitude to account 

 for movement of water to the top of the plant can develop in them without 

 a complete loss of turgor by the leaf cells. If the cell is in direct contact 

 with one of the branches of the xylem ducts which ramify throughout the 

 lamina of the leaf, water will move from the vessel or tracheid into the cell; 

 this results in the development of tension in the water column terminating 

 in this xylem element. The diff^usion pressure deficit of the water in the 

 conducting elements will be increased by the amount of this tension. Water 

 under a tension ("negative pressure") of lO atmos., for example, has a dif- 

 fusion pressure just lO atmos. less than that of pure water at the same tem- 

 perature which is not under tension (Chap. XI). In other words its dif- 

 fusion pressure deficit is lO atmos. If the cell from which evaporation is 

 occurring is not in direct contact with a xylem element a gradient of diffusion 

 pressure deficits, gradually increasing in magnitude from cell to cell in the 

 direction in which the water is moving, is established between the xylem 

 element and the cell from which evaporation is occurring. 



The tension developed in the conducting elements is transmitted along 

 their entire length to their lower termination just back of the root tips and 

 probably, very often at least, across the root tissues as well. The magnitude 

 of the tension which develops in the xylem conduits is increased by conditions 



