Grafts et al. — 178— Water in Plants 



Water movement takes place in the non-living xylem elements along a hydrostatic 

 gradient developed by transpiration pull. The water columns are in a state of tension 

 much of the time, a state made possible by the forces of cohesion (internal pressure) 

 between the water molecules and adhesion between water and the cellulose walls. When 

 such columns break, bubbles of water vapor form and the elements occupied by them 

 are rendered ineffective in water transport. When tension is reduced, such bubbles 

 may shrink and collapse, mending the columns. 



Studies on the xylem have been made using dyes, freezing, transpiration measure- 

 ments, and direct microscopic observation. These studies largely confirm the co- 

 hesion mechanism. Many methods have been used to estimate tensions in the xylem; 

 potometer methods, measurement of "suction tension", and observation of differences 

 in refractive indices have proved useful. Periodic measurement of the concentration 

 of solutions injected into the petioles of squash leaves has given a view of changes in 

 water status of the plants. 



Many methods have been used to study rates of water movement in plants ; com- 

 parison of the weight of water lost with the transverse area of conducting elements 

 gives an over-all average rate. Indicators, including salts, dyes, radioactive materials, 

 and heat have proved valuable; rates up to several meters per hour have been meas- 

 ured. Rates of flow vary with water supply, transpiration, and with structural varia- 

 tions in the plant. 



Water balance in plants varies and with it water distribution. Redistribution be- 

 tween leaves, fruits, trunk, and roots takes place under fluctuating stress. Young 

 leaves usually exceed old in their ability to absorb and hold water ; sun leaves exceed 

 shade leaves. 



Translocation of water and solutes in the plant are correlated. Minerals absorbed 

 from the soil travel upward in the xylem with the transpiration stream. Foods syn- 

 thesized in the leaves move downward to the roots and upward to growing parts and to 

 developing fruits through the phloem. Excess minerals, with the possible exception of 

 calcium, may move out of leaves via the phloem. 



Two main theories on the mechanism of food movement are 1) the protoplasmic 

 theories postulating movement in, on, or along the surface of the sieve-tube protoplasm 

 independent of water and accelerated by metabolic energy; 2) mass flow, picturing 

 a simultaneous movement of foods and water in solution through the lumina of the 

 sieve tubes along a gradient of hydrostatic pressure developed osmotically. Sieve tube 

 elements are assumed to have a ready permeability for such flow and the solution under 

 pressure is maintained in phloem strands by sheaths of living parenchyma cells. 



Current research on translocation indicates that sugars, nitrogen compounds, 

 fluorescein, radiophosphorus, the flower-inducing hormone, phloem-limited viruses, 

 thiamin, pyridoxine, pantothenic acid, riboflavin, auxin, and 2,4-dichlorophenoxyacetic 

 acid may all be transported in the phloem and some experiments indicate that two or 

 more are commonly carried along together. The simplest explanation for such move- 

 ment is the mass-flow mechanism which involves a recirculation of water and entails 

 a close correlation between the functioning of the xylem and phloem. 



