242 THE MOVEMENT OF WATER THROUGH THE PLANT ■ 



is transported through plants, be they the tallest of trees or herbs only a few | 

 feet in height. It is also generally agreed that "root pressures" play a minor 

 role in this process at least under certain conditions and in some species- 

 There is no doubt that living cells are involved in the phenomena generally 

 described under the term "root pressure" and there is at least a possibility 

 that living cells of the xylem are in some way essential to the maintenance 

 or operation of the cohesion mechanism. 



Rates of Movement of Water through Plants. — The rate which water 

 ascends through the xylem ducts may vary from a movement so slow as to be 

 almost imperceptible to a speed of at least 75 cm. per minute (Huber, 1932). 

 By rate is meant the length of the water column which will move past a given 

 point in one minute. In general daily variations in the rate of ascent of water 

 closely parallel daily variations in transpiration rate (Baumgartner, 1934)- 



Lateral Movement of Water. — A cell to cell lateral movement of water 

 in a radial direction undoubtedly occurs along the vascular rays in the stems 

 of most species of plants. In woody stems there is probably also a lateral 

 movement of water around the stem in a tangential direction. Except in 

 trees in which the "grain" of the wood is twisted, the conducting vessels on 

 one side of the tree generally connect with branches on that side of the tree 

 at their upper extremity, and with roots on the same side of the tree at their 

 lower extremity. If no lateral movement of water occurred in woody stems, 

 it would be expected that removal of the roots from one side of a tree would 

 result in a dearth of water, or perhaps even death of the leaves or branches 

 on that side of the tree. 



Experiments have been performed on apple, peach, oak, and other woody 

 species in which the roots on one side of the plant were removed in an attempt 

 to determine whether or not lateral movement of the water occurs (Auchter, 

 1923). Although the water content and growth of the plants treated in 

 this manner diminished there was no difference in the moisture content of the 

 leaves on the two sides of the tree. Neither did the leaves on the side from 

 which the roots had been removed show any greater tendency to wilt on clear, 

 warm days than those on the other side. These results indicate very strongly 

 that lateral movement of water occurs in woody stems, and that the water 

 conductive system of plants acts as a unit system. 



Downward Movement of Water. — There are a number of experiments 

 on record which indicate that downward movement of water can occur in 

 stems. For example, Dixon ( 1924) found that if the tip of the leaf of a potato 

 plant be cut off under an eosin solution the liquid will descend through the 

 xylem tissue of the leaf, petiole, and stem, and eventually reach the under- 



