TRANSPIRA TTON. 



45 



S7. Lifting power of transpiration, -Not only clofs transpiration go on 



quite intiojjenrlunth" of root pressure, as we ha\e rlisco\'ered from other 



experiments, bat transpiration is eapable of exerting a 



lifting ]jo\ver on the water in the plant. This may 



be demonstrated in the following wav: Place the cut 



end of a leafy shoot in one end of a U tube and lit it 



water-tight. Partly fill this arm of the U tube with 



water, and add mercur^• to the other arm until it 



stands at a level in the two arms as in fig. 54. In a 



short time we note that the mercury is rising in the 



tube. 



88. Hoot pressure may exceed transpiration. — If we 



cover small activeh' growing plants, such as the pea, 



corn, wheat, bean, etc., with a bell jar, and place them 



in the sunlight where the temperature is suitable for 



growth, in a few hours, if conditions are fa\'orable, 



we shall see that there are drops of water standing out 



on the margins of the leaves. These drops of water 



have exuded through the ordinary stomata, or in 



other cases what are called water stomata, through 



the inlluence of root pressure. The plant being covered by the glass jar, 



the air soon becomes saturated with moisture and transpiration is checked. 



Root pressure still goes on, howe\'er, and the result is shown in the exuding 

 drops. Root pressure is here in excess of transpiration. 

 This phenomenon is often to be observed during the sum- 

 mer season in the case of low-gro«-ing plants. During the 

 bright warm day transpiration 

 — " ^ equals, or may be in excess of, 

 pjg ,,^ root pressure, and the leaves 



Estimation of the amount of are consequently tlaccid. .\s 



Fig. 54. 

 Experiment to 

 show lifting power of 

 transpiration. 



transpiration. The tubes are 



nightfall comes on the air 



filled with water, and a.s the 



water tr.inspires from the leaf becomes more moist, and the 



surface its movement m the tube 



from a to 6 can be measured, conditions of light are such 



(.ifter.Mangin.i ^j^^ ^^^^ transpiration is les- 



sened. Root pressure, however, is still active because the soil is still warm. 

 In these cases drops of water ma\" be seen exuding from the margins of the 

 leaves due to the excess of root pressure over transpiration. Were it not 

 for this proWsion for the escape of the excess of water raised by root pres- 

 sure, serious injury by lesions, as a result of the great pressure, might 

 result. The plant is thus to some extent a self-regulatory piece of 

 apparatus so far as root pressure and transpiration are concerned. 



89. Injuries caused by excessive root pressure. — Some varieties of toma- 

 toes when grown in poorly lighted and poorly ventilated greenhouses suffer 



