SUMMARY. 75 



closure the rate of increase again becomes slow and is further re- 

 tarded during the night. 



2. Changes of movement caused by factors other than light are 

 not necessarily accompanied by corresponding changes in the starch- 

 content of the guard-cells. 



3. Reduction of light to less than half of normal is usually 

 necessary to produce any effect upon the stomata of plants growing 

 in the open. When the stomata are closing, they respond to de- 

 crease of light more rapidly than when opening. 



4. Stomata open at night as a result of moonlight or a strong 

 artificial light of much less intensity than 1 per cent of the sunlight 

 maximum. They open more readily toward morning than before 

 midnight, and hence at night also the reactions which tend to pro- 

 duce opening or closure are more readily hastened than reversed. 



5. The temperature of the air affects the speed at which the 

 stomata open during the morning. The length of time required 

 for opening is reduced by approximately one-half for every 10 C. 

 rise in temperature. This, of course, occurs only within the limits 

 of temperature at which protoplasm functions. 



6. When the temperature of the soil rises too much the stomata 

 close, and in extreme cases the plant wilts. 



7. The temperature of the leaves was usually found to be lower 

 than that of the air when the stomata were open, and higher when 

 closed in sunlight. 



8. A high humidity of the air permits the stomata to open 

 wider and remain open longer than a low humidity under most 

 conditions. This is especially true when the water-content has 

 decreased seriously and to a point where the plant has great diffi- 

 culty in obtaining sufficient water to meet evaporation during the day. 



9. No atmometer or evaporimeter was found that would measure 

 at all accurately the effect upon the plant of all the factors con- 

 cerned in evaporation. 



10. When the leaves of a plant were wet by dew or rain, or wet 

 artificially, the stomata usually opened if closed, or opened more 

 widely if partially open. When the water dried the stomata closed 

 wholly or partially. 



11. Wind caused increases of transpiration unlike the increases 

 of evaporation as measured by atmometers. In the main, the plant 

 showed much less response to wind than the atmometer, but with 

 the sudden advent of a high wind it often showed greater response. 



12. Wind carries dust to the leaves, coating them and often 

 wedging particles into the open stomatal slits, so that they remain 

 permanently open. 



13. Water-content is the chief of the factors that determine the 

 rate at which the leaves can be supplied with water. When the 

 soil is dry the rate of supply is slow, causing turgor to be lost early 

 in the day and the stomata to close. 



