228 TEXTBOOK OF BOTANY 



mitted through the water column in the tube to the mercury, and the 

 mercury is pulled upward. 



If the apparatus is carefully constructed and all air removed from 

 the water, the mercury column may be readily pulled to a height of 120 

 to 130 cm. When the cups were coated with gelatin and the size of the 

 pores was thus reduced, a height of 226.6 cm. was attained. Mercury is 

 13.6 times as heavy as water, and when evaporation of water lifts mer- 

 cury 226.6 centimeters it is equivalent to lifting water 100 feet. The 

 height to which the mercury is lifted in such experiments is limited by 

 the entrance of air into the cup or tube through minute pores. The 

 tensile strength of water enclosed in tubes is equivalent to at least 150 at- 

 mospheres, and the energy of molecular motion is sufficient to lift col- 

 umns of water many times the height of the tallest trees. 



When twigs of arbor vitae, red cedar, or box elder are used in place 

 of the porous cup, transpiration may raise mercury columns 90 to 101 

 cm. in height before enough air enters the tube to terminate the demon- 

 stration. This is equivalent to a column of water 40 to 50 feet in height. 

 During the experiments from which the above figures were obtained 

 the height of the barometer varied from 73 to 75 cm. of mercury. Does 

 barometric pressure account for the rise of water in these experiments? 



Water-vapor gradient. Next to the opening and closing of the stomates, 

 the most important factor influencing transpiration is the condition of 

 the water vapor inside and outside the leaf. If the concentration of 

 water vapor inside the intercellular spaces of the mesophyll is greater 

 than it is outside the leaf, there will be a gradient in the diffusion of 

 vapor from the cells of the leaf to the outer atmosphere. This gradient 

 is augmented by the relative rates of molecular motion whenever the 

 temperature of the leaf is higher than that of the atmosphere. The 

 gradient may be steep if the air outside is very dry and the air inside 

 nearly saturated, and the temperatures of the leaf and the air are the 

 same. Under these conditions the concentration of water molecules is 

 much greater in the air spaces of the leaf than in the atmosphere. If the 

 temperature of the leaf becomes higher than that of the atmosphere the 

 gradient will be increased because within the air spaces the number of 

 free molecules is increased as well as their rate of movement. If the 

 temperature of the leaf becomes lower than that of the atmosphere the 

 gradient will be reduced because the number of free molecules within 

 the air spaces and their rate of movement are decreased. 



Let us assume that during the course of a clear warm morning in 



