Chapter X — 205 — Loss and Retention 



eosin by these shoots was explained on the basis of internal water adjust- 

 ments and certain observed stimulative effects of added salts seem best 

 explainable on their action on the osmotic nature of the cells of the shoot. 



In later work, Dixon (1938^7) and Dixon and Barlee (1940) satu- 

 rated test plants by submersion for 24 hours and still found that eosin rose 

 from 15 to 69 centimeters. Oxygen and light favored the movement which 

 was very slight in the dark. 



Crafts (1939t/) confirmed the fact that subaqueous transpiration oc- 

 curs only in the light and suggested that rise of water results from osmotic 

 uptake by the phloem according to the mechanism of Munch (1930). In 

 the light photosynthesis in leaves and utilization of foods in the growing 

 and respiring or storing cells causes a transport of osmotically active solutes 

 via the phloem. By mass flow this would be accompanied by a certain move- 

 ment of water. When one branch of a forked Syringa vulgaris stem was 

 ringed and the whole shoot submerged, upward movement of eosin was 

 more rapid in the xylem of the unringed branch in which, presumably, some 

 phloem transport was taking place. 



The importance of this work would seem to justify more research, for 

 if subaqueous transpiration can be satisfactorily explained without resort 

 to a secretory mechanism, upward movement of water through the xylem 

 may be accounted for on a physical basis. On the other hand, if water 

 secretion is involved, experimentation in this field is greatly complicated, 

 for such an activity is difficult to measure and control. For anyone inter- 

 ested in this work, it is suggested that chlorotic or albino plants may prove 

 useful in an experimental approach to the problem. 



Conclusions : — In the tremendous literature on transpiration, only a 

 portion of which has been presented here, the emphasis has been largely 

 on the effect of purely physical factors upon rate and volume of water loss. 

 And, though physiological processes have been studied in relation to trans- 

 piration, the active functions of plant cells have received little attention. 

 Likewise the structure and unique nature of water have been neglected. 



The high surface tension of water and the hydrophilic nature of cell 

 walls and protoplasm are essential to the development of transpiration pull 

 in leaves. The strong coordinating forces that cause water to adhere to the 

 conducting elements are vital to the maintenance of the continuous water 

 columns. The strong bonding forces between water molecules and solutes 

 enable the cells to retain their aqueous contents in competition with desiccat- 

 ing factors of the soil and atmosphere. It seems that the very ability of 

 plants to leave their aquatic environments and invade the land depends not 

 alone upon the adaptive nature of the plants but also upon the peculiar char- 

 acter of the water too. 



The active control of water by plants is a fairly new subject of in- 

 vestigation. Though the "vital" forces involved may elude direct study, 

 they must all be explainable eventually in terms of physical and chemical 

 laws. Because of the intimate relation between such forces and the proces- 

 ses of metabolism, studies on cell energy relations should offer an advan- 

 tageous point of attack. In this relation it is encouraging that many cur- 

 rent researches in plant physiology are concerned with active processes of 

 water and salt absorption. 



The essential role of water involves its contribution to the stature and 

 elemental composition of the plant, its place in metabolism, its function in 

 the congregation of solutes at the absorbing surfaces of the roots, and its 



