COMPARATIVE TRANSPIRATION OF TOBACCO 



AND MULLEIN. 



J. D. Sayre. 



Comparative studies of the transpiration rates of Tobacco 

 and Mullein have been carried on during the winters of 1917 

 and 1918 in the Ohio State University Botanical Greenhouse 

 and Laboratory under the direction of Dr. E. N. Transeau. 

 These experiments were performed with special apparatus 

 designed by him and described in the Botanical Gazette, Vol. 52, 

 on pages 54 to 60. By means of this apparatus it is possible to 

 obtain a continuous record of the water loss from sealed potted 

 plants. As many as six different plants can be used at one 

 time. 



Continuous records of the common environmental factors 

 influencing transpiration were also obtained by the use of record- 

 ing instruments; temperature, humidity, duration of sunshine, 

 evaporation and wind velocity. From these records of transpira- 

 tion from the plants and of variations in the environmental 

 factors, certain conclusions have been drawn concerning the 

 relation of the hairy coverings on the mullein leaves to the 

 resistance of the plant water loss, the relation of the stomata to 

 transpiration, and the daily rhythm of the transpiration curve. 

 Figure 1 shows the results of one of the experiments which was 

 performed in the greenhouse. These curves show the method 

 of expressing the results. The details and records are contained 

 in a paper to be published elsewhere. 



When compared with a similar thin-leafed tobacco plant 

 {Nicotina sp.) the following conclusions regarding the resistance 

 of the mullein leaves ( Verbascum thapsus) to transpiration were 

 obtained. 



1. Mullein leaves offer greater resistance to water loss in 

 darkness than in light. 



2. Mullein leaves offer less resistance to water loss in wind 

 than in still air. 



3. Mullein leaves respond as much or more to changes in 

 the environment as to tobacco leaves. 



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