TRANSPIRA TION. 



45 



87. Lifting power of transpiration. Not only does transpiration go on 

 quite independently of root pressure, as we have discovered from other 

 experiments, but transpiration is capable of exerting a 



lifting power on the water in the plant. This may 

 be demonstrated in the following way: Place the cut 

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

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

 water, and add mercury 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. Boot pressure may exceed transpiration. If we 

 cover small actively 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 favorable, 

 we shall see that there are drops of water standing out 

 on the margins of the leaves. These drops of water 



Fig. 54. 



Experiment to 

 have exuded through the ordinary stomata, or in show lifting power of 



. ,, , transpiration. 



other cases what are called water stomata, through 



the influence 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, however, 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-growing plants. During the 

 bright warm day transpiration 



"" "^"^^""" !I " "" ' equals, or may be in excess of, 



pjg ss> root pressure, and the leaves 



Estimation of the amount of are consequently flaccid. As 

 transpiration. The tubes are . . c n *u 



filled with water, and as the nightfall comes on the air 

 water transpires from the leaf becomes more moist, and the 

 surface its movement in the tube 

 from a to b can be measured, conditions of light are sxich 



(After Mangin.) 



also that transpiration is les- 



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

 In these cases drops of water may be seen exuding from the margins of the 

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

 for this provision 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 



