DISCUSSION QUESTIONS 363 



general, as with the apple tree previously described, the rate of photosynthesis 

 under these conditions showed a close correlation with the light intensity. 



When the daily cycle of photosynthesis is measured in small plants, i.e. 

 under such conditions that light is a limiting factor for little or none of the 

 leaf area, the resulting curves often have a different shape from that shown 

 in Fig. 93, even on clear days. Often the peak of the curve occurs during 

 the mid-morning hours (Kostychev et ah, 1926, and others) ; sometimes the 

 rate reaches a maximum during the forenoon, decreases to a minimum dur- 

 ing mid-day, and rises to a secondary maximum later in the afternoon, before 

 falling again to zero (Kurssanow, 1933). The explanation for daily photo- 

 synthetic cycles of the types just described is not clear but they are probably 

 due to a temporary limiting effect of certain internal factors, such as accumula- 

 tion of the products of photosynthesis, or to temporary mid-day closure of the 

 stomates (Chap. XIII). 



Discussion Questions 



1. What environmental factors would be of first and second Importance as 



limiting factors in photosynthesis outdoors in the east central United States 

 in July? in January? the year round? In a greenhouse in the same region 

 in July? in January? In the semi-desert regions of the southwest, con- 

 sidered the year round? 



2. If in the morning a given rate of photosynthesis were found to occur in a 



plant under favorable conditions of light, CO2 concentration, temperature, 

 and soil water supply, would you expect to find the same rate in the after- 

 noon if all environmental factors remained unchanged? Explain. 



3. If half the leaves were removed from a plant how will the daily rate of 



photosynthesis per unit of leaf area of the remaining leaves compare with 

 the rate if the leaves had not been removed? Explain. Would the distribu- 

 tion of the removed leaves on the plant make any difference? 



4. Explain in some detail why, other conditions being favorable, a plant grow- 



ing under conditions of a favorable soil water supply will accomplish more 

 photosynthesis in the course of a day than one growing where the soil 

 water supply is inadequate. 



5. Would increasing the intensity of the sunlight falling on a corn plant by 



the use of mirrors at noon on a cloudless day result in any appreciable 

 increase in the rate of photosynthesis? Explain. 



6. Under what conditions would the addition of water to the soil around a 



growing plant be expected to result in an appreciable increase in the rate 

 of photosynthesis? no very great change? a measurable decrease? 



7. How would you expect the rate of photosynthesis to behave in plants sub- 



jected to continuous Illumination? 



8. If the CO2 concentration of the atmosphere is usually the limiting factor 



during the summer months ior plants well exposed to light, why is it pos- 

 sible to Increase the production of many crop plants by adding fertilizers 

 to the soil? 



9. Why does adding carbon dioxide to the atmosphere of greenhouses In the 



northeastern United States during the winter months often have little or no 

 beneficial effect on plants? 



