Crafts et al. — 140 — Water in Plants 



tions have been implied by various investigators with respect to the general 

 processes of absorption by roots (van Overbeek, 1942 ; White, 1942) ; 

 to transpiration (Dixon and Barlee, 1940) ; and to translocation (God- 

 LEWSKi, 1884; Janse, 1913; and Bose, 1923). These will be considered 

 in the two chapters that follow. 



Some who have studied mechanisms of cell elongation, or cell growth, 

 have been inclined to suggest processes involving active control of water. 

 The theory that auxin acts to increase wall plasticity and promote active 

 deposition of wall substance has many adherents, notably Heyn (1940). 

 Evidence that plastic extensibiHty in potato tuber tissue is greater under 

 aerobic conditions than where oxygen is excluded has been supplied by L. 

 and M. Brauner (1943a). Others believe that auxin enhances the im- 

 bibition pressure of the protoplasm. Strugger (1934), for example, found 

 by plasmolytic methods that an increase in protoplasmic viscosity accom- 

 panied cell enlargement, and concluded that an enhanced sweUing pressure 

 is directly involved. He suggested that acids resulting from respiratory 

 activity might promote swelling. Other explanations of cell enlargement 

 include an increase in solute concentration and the existence of electrical 

 potentials across the cell membranes produced by auxin, either of which 

 might bring about increased turgor, resulting in growth. 



Growth is a complicated process. Perhaps several of the above mechan- 

 isms are operating simultaneously, all influenced by auxin directly or in- 

 directly. The problem is one of the most difficult in physiology ; neverthe- 

 less there has been much progress. 



The striking movements exhibited by some plants have been of consid- 

 erable interest to physiologists. The sudden drooping of leaflets and 

 petioles of Mimosa pudica upon stimulation has caused much speculation 

 as to the cellular mechanics involved. It is generally agreed that the re- 

 sponse is the direct result of the loss of turgor by the cells comprising the 

 lower region of the pulvinus. This may be due to an active contraction of 

 the protoplast (Bose, 1928; Weidlich, 1930) whereupon liquid is secreted 

 into the intercellular spaces. Bose has been able to identify the contracting 

 cells by staining methods, and claims to have found by actual measurement 

 a decrease in cell diameter due to the contraction. 



At the same time, the cells comprising the upper region of the pulvinus 

 expand somewhat. This is due in part to stretching resulting from the 

 force of gravity on the drooping leaf, and an attendant increased DPD, 

 which causes an additional uptake of water (Weidlich, 1930). During 

 the recovery process, which is of considerably longer duration than the 

 initial response, the cells in the lower part of the pulvinus apparently re- 

 absorb liquid and regain their normal turgor. That this liquid is relatively 

 pure water, and that the contraction is not a result of changes in permeability 

 which would permit whole cell sap to flow out, is the view of Weidlich 

 (1930). 



Glandular secretion is another process in plants poorly understood at 

 the present time. According to Blackman (1921), Pfeffer in 1877 pro- 

 posed three hypotheses to account for the exudation of fluid from living 

 cells : 



1) An unequal osmotic pressure developed by the membrane in different parts of 

 the cell, 



2) Unequal distribution of osmotic material in different parts of the cell, 



3) Presence of osmotic substances in cell wall outside of the membrane, causing 

 water to move out of the cell. 



