Crafts et al. — 160 — Water in Plants 



must occur in a stem subjected to such rapid cooling. Expansion during 

 the formation of ice in the outer part of a stem would cause local strains on 

 the vessels nearer the center causing rupture in the liquid. Furthermore, 

 disturbance of forces of cohesion at the liquid: crystal interface as ice is 

 formed might result in rupture. 



Transpiration Experiments: — In 1914 Bakke performed a series of 

 experiments on sunflower plants that were lifted from the soil and brought 

 into the laboratory to wilt. He made continuous transpiration experiments 

 using cobalt chloride indicator paper and found a sharp rise in transpiration 

 rate just beyond the permanent wilting condition. He interpreted his re- 

 sults as indicating that wilting occurs at a definite point and permanent 

 wilting represents the most advanced stage of wilting possible without seri- 

 ous rupture of the water columns in the xylem. Continuing the work in 

 1918, Bakke found the stomatal movements not to be important during 

 advanced wilting. During progressive wilting there was a gradual drop in 

 transpiration, followed by a plateau of no change and then an abrupt rise in 

 rate. The plateau was considered to represent a state of equilibrium be- 

 tween the plant and its environment representing the maximum tension 

 that the plant could develop. The rise at the end of the permanent wilting 

 range represented serious rupture of the water columns. When this rup- 

 ture was complete the plant could not be expected to recover when moisture 

 was added to the soil. 



From a priori reasoning Knight (1922) decided that as a plant passes 

 from a state of turgor to a wilted condition, there is no reason to expect that 

 at any particular stage in the wilting process the water columns would sud- 

 denly rupture. He pictured replacement of water by gas as a gradual 

 process during wilting, the water columns being severed one by one until 

 enough are broken to bring about failure in the supply to the leaves. Work- 

 ing with detached shoots in potometers. Knight made simultaneous meas- 

 urements of stomatal opening and transpiration during progressive wilting. 

 Wilting was induced by closing the stopcock of the potometer, thus stopping 

 the water supply. There was a sharp increase in stomatal opening accom- 

 panied by an increase in transpiration during early wilting; this was fol- 

 lowed by a gradual drop. The increase however did not correspond with 

 that observed by Bakke, as it occurred early in the wilting process. Ob- 

 jection could be made to the use of detached shoots in such experiments for 

 reasons mentioned earlier. 



Direct Observation of Xylem : — Numerous direct observations have 

 been made on the xylem of plants to determine the presence or ab- 

 sence of gas in the conducting tracts. Among the first to use this method 

 were Vesque (1883) and his student Capus (1883). Unfortunately their 

 method of preparing their material by cutting and peeling undoubtedly 

 caused formation of vapor phase in every element whose water was under 

 tension. Their observations were largely responsible for the persistence of 

 the idea that there were liquid-gas chains (Jaminian chains) in xylem con- 

 ductors. 



HoLLE (1915) and Bode (1923) made critical observations on the state 

 of water in the xylem of plants. They concluded that even under conditions 

 of extreme wilting there were continuous intact columns of water in the 

 xylem. Conclusions of earlier workers that gas was present were found 

 to be the result of faulty preparation and observation. Bode used a hori- 



