Grafts et al. 



— 204- 



Water in Plants 



water movement to the highest tree tops. Such movement is not generally 

 considered to utilize respirational energy except as it is required to maintain 

 the normal structure of the cells. 



The first demonstration of vi^ater movement through submerged plants 

 v^as made by Unger in 1862 (Thut, 1932b). In this experiment a Pota- 

 mogeton crispus plant was placed with its roots in one vessel and its tops 

 in another with both submerged. In eight days the vessel containing the 

 tops had gained 1.6 grams. Similar experiments on water movement in 

 submerged plants were performed by Snell (1886), Sauvageau (1891), 

 HocHREUTiNER (1896), DixoN (1898), Pond (1903), and Thoday and 

 Sykes (1909). Such experiments utilized rooted plants, and cut shoots. 



Experiments on rooted plants (Unger, 1862; Thut, 1932&) need not 

 be considered here for, as Thut has explained, root pressure may account 

 for the water movement. Observations of water movement through sub- 

 merged cut shoots are of great interest here for such movement cannot be 

 caused by root pressure nor by transpiration pull. 



Dixon, the champion of the cohesion 

 theory through many stormy years, was 

 forced to resort to water secretion to explain 

 subaqueous transpiration (Dixon, 1898, 

 1914, 1938&; Dixon and Barlee, 1940). 

 Secretion was called on to account for the 

 rise of eosin through the xylem in the ex- 

 periment illustrated in Figure 56 (Dixon, 

 1938a). 



Thoday and Sykes (1909) attached 

 small glass bulbs containing eosin to the cut 

 ends of submerged branches of Potamoge- 

 ton luccns and found a rapid rise (5.7 to 

 9.5 cm. per minute) of eosin in the stems. 

 When the apex of the branch was removed, 

 movement slowed down and when the leaves 

 were removed little or no movement oc- 

 curred. From these observations they con- 

 cluded that the leaves were the organs responsible for movement. 



Smith, Dustman, and Shull (1931) criticised the early experiments 

 of Dixon and others, maintaining that proper precautions were not taken 

 to ensure complete water saturation. They argued that the ascent of the 

 transpiration stream is caused by evaporation from cell wall colloids of the 

 mesophyll and is not dependent upon secretion or respirational energy. 

 When they took proper precautions to completely saturate their experi- 

 mental plant materials they found no rise of eosin into completely sub- 

 merged shoots. 



In a later paper, Shull (1939) answers van der Paauw's (1935) 

 criticism, that adsorption of eosin might have been responsible for Smith, 

 Dustman, and Shull's negative results, by the postulation that slow solu- 

 tion of gases might explain water movement in his (van der Paauw's) 

 experiments. Neither advanced any support for Dixon's secretion theory. 

 Nor was Wilson (1947) able to demonstrate the existence of continued 

 movement of the material in a transpiration stream in completely sub- 

 merged cut shoots of Ranunculus fluitans. Some short time absorption of 



Fig. 56. — Experimental con- 

 ditions for demonstrating sub- 

 aqueous transpiration. Redrawn 

 from Dixon (1938a). 



