212 FACTORS AFFECTING TRANSPIRATION 



Discussion Questions 



1. Why does the equilibrium vapor pressure of water increase markedly with 



rise of temperature? 



2. What factors determine the vapor pressure of an atmosphere? Why does 



water-vapor diffuse from a region of greater to a region of lesser vapor 

 pressure? 



3. If a pan of water is maintained at a temperature of 30° C. and exposed to 



the outside atmosphere when would it evaporate most rapidly, in the sum- 

 mer or in the winter? 



4. Plot curves which might reasonably be expected to represent the daily varia- 



tion in the vapor pressure of an atmosphere: (a) out-of-doors on a per- 

 fectly clear summer day, (b) out-of-doors on a perfectly clear winter 

 day, (c) in a closed greenhouse during a clear day, (d) out-of-doors on a 

 clear late spring day with a heavy dew in the morning, and (e) out-of- 

 doors during a summer day upon which a heavy thunderstorm occurs in 

 the afternoon. 



5. Why is the air of the California and Arizona semi-deserts considered "dry" 



when it contains approximately the same quantity of water per unit volume 

 as the "moist" air of the Minnesota lake country? 



6. Cell A has an osmotic pressure of 30 atmos. and wall pressure of 15 atmos. 



Cell B has an osmotic pressure of 30 atmos. and a zero wall pressure. 

 Cell C has an osmotic pressure of 30 atmos. and the water within it is 

 under a tension of 15 atmos. From which of these cells will water 

 evaporate most rapidly? least rapidly? Explain. 



7. Explain exactly why an increase in leaf temperature usually results in an 



increase in the rate of transpiration. 



8. If half the leaves were removed from a plant what effect would this have 



on the rate of transpiration from the rest of the leaves? Would the 

 effect be the same under all environmental conditions? 



9. Which of the three factors, vapor pressure, sunlight or air temperature do 



you think usually has the greatest influence on transpiration rates under 

 field conditions? Which second? Why? What fourth environmental 

 factor is often of greater importance than any of these three? 



10. Under what conditions would transpiration be approximately proportional 



to the vapor pressure deficit of the atmosphere? When not? 



11. Assuming lOO per cent relative humidity in the intercellular spaces, open 



stomates, and sufficient air movement to prevent appreciable accumulation 

 of water-vapor around the leaves, which plant will lose water-vapor more 

 rapidly — one in an environment of 80 per cent relative humidity and 40° C. 

 temperature, or a similar one in an environment of 80 per cent relative 

 humidity and 20° C. temperature? Explain. 



12. Making the same assumptions as in question il, which plant will transpire 



more rapidly — one with a leaf temperature of 35° C. in an atmosphere of 

 30° and relative humidity of 70 per cent or one with a leaf temperature of 

 30° C. in an atmosphere of 30° C. with a relative humidity of 60 per cent. 

 Explain. 



13. Making the same assumptions as in question 11, which would have the greater 



effect on the rate of transpiration — an increase in leaf temperature from 

 30 to 35° C. or a decrease in atmospheric relative humidity from 80 to 60 

 per cent, air temperature remaining at 30° C. ? Explain. 



14. The transpiration of a potted tobacco plant was measured hourly during a 



