to the Study of Transpiration. 



441 



the other three tubes set at 7°, 8°, and 9°. Should No. 2 show 

 dew at 7°, No. 3 none at 8°, the dew-point is between these 

 temperatures, and is found at the next move within one or 

 two-tenths of a degree. When two tubes stand side by side, 

 one at a constant temperature of 7°'l, dimmed with moisture, 

 while the other at 7°*2 is free, the dew-point is known with 

 certainty within one-tenth of one degree. The thermometers 

 are graduated to tenths, and of course have been carefully com- 

 pared. 



The following experiment, made in the course of a study on 

 the water-changes in a young bean-plant during sleep-move- 

 ments, may be of interest as illustrating the workings of the 

 method. 



Plant, a bean-seedling with 10 leaves, in pot wrapped with 

 rubber and placed under bell-jar of 8 lr. capacity. Heavy 

 clouds, dull light. Temperature in bell-jar throughout 20° -25 

 C. Draft, 972 dr 5 CC per minute. 



Time. 



Dew. 



No dew. 



Conditions, etc. 



11 : 23 a. m. 



8°'5 C. 



8°-8 C. 



Leaves erect. Plant now 

 first darkened. 



11:27 



Not found. 



8°'l 





11:30 



« 



7°'9 



Little or no drooping. 

 11 : 38, lower leaves have 

 declined by 45°. 



11:42 



6°- 



6°-3 





11:53 



No change. 



No change. 



Plant half asleep. 



12:05 



'it 



a 



No chanere. 



The plant manifested nyctitropic movements only slowly, 

 and never completely assumed a sleeping position. Diminu- 

 tion in the rate of transpiration was indicated in 4 minutes 

 after change of light, by a fall of at least o, 4 in the dew- 

 point; in 7 minutes by a fall of over o, 6 ; and in 19 minutes 

 a nearly constant transpiration at a lower rate was definitely 

 ascertained, giving a dew-point between 6°* and 6° # 3. As 

 exact weights of water exhaled were not sought, a nearer 

 determination of the dew-point was not made. 



Botanical Laboratory of Harvard University, 

 May 4, 1898. 



