230 THE MOVEMENT OF WATER THROUGH THE PLANT 



conduction of water occurred as a result of periodic changes in the osmotic 

 pressure of the living cells present in the xylem (especially the woo J ray cells). 

 Upon increase in their osmotic pressure water was supposed to move into the 

 cells from bordering xylem elements. Upon decrease in osmotic pressure water 

 was supposed to move back into a xylem duct. In the xjlem duct it was 

 supposed that water would move upward until it came in contact with another 

 living ray cell because of a lower air pressure in the upper than in the lower 

 part of the vessels. The ascent of water in stems was thus supposed to be 

 brought about by a number of repetitions of this same process. There is little 

 or no direct evidence in support of this or any of a number of more or less 

 similar theories which have been suggested. 



The experiments of Strasburger (1893) demonstrated quite clearly that 

 the primary mechanism of the rise of sap in trees operates independently of 

 the living cells of the stem. He performed many experiments upon the 

 movement of water through woody stems in which the living cells had been 

 killed by one method or another. In one experiment, for example, he used a 

 75-year-old oak tree about 22 meters in height. This was sawed off close to 

 the ground and the cut end transferred to a solution of picric acid, which is 

 toxic to living cells. The picric acid solution slowly moved up the stem. 

 Fuchsin, added to the liquid in which the base of the tree was immersed 3 days 

 after the picric acid, also ascended to the top of the tree through tissues in 

 which the living cells had been killed by the picric acid. Water also con- 

 tinued to ascend through similar stems after they had been completely killed 

 by exposure to a temperature of 90° C. 



Similar experiments have been performed by later investigators, especially 

 Roshardt (1910), Overton (1911), and MacDougal, et al. (1929)- AH 

 of these investigators have confirmed Strasburger's results that water would 

 continue to ascend for some time through stems, segments of which had been 

 exposed to one treatment or another which would kill all living cells present. 



In all experiments of the type just described, it has been observed that the 

 leaves at the top of a stem, part or all of which has been killed, sooner or 

 later wilt and wither, although this effect is by no means an immediate one, 

 often appearing only after several days. Proponents of the vital theories have 

 accepted this as evidence that the living cells of a stem are essential for the 

 passage of water through it. Dixon (1914) however, considers that this 

 delayed lethal effect on the leaves is due to either or both of two causes. 

 Killing of the stem tissues often causes the formation of substances which plug 

 up the vessels or tracheids, thus impeding the upward movement of water. 

 Furthermore, death of the cells in the treated regions causes the release into 

 the conducting channels of toxic compounds which, when transported to the 



