40 



PHYSIO LOG Y. 



88. Root 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 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 have exuded through the ordinary stomata, or in 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 mois- 



Fig. 38. 

 Estimation of the amount of ture and transpiration is checked. 



transpiration. The tubes are -o^^^ nrpusnrp still o-nps nn how 



filled with water, and as the ^^°°' pressure stm goes on, now- 



water transpires from the leaf ever, and the result is shown in 

 surface its movement in the tube 



from a lo b can be measured, the exudmg drops. Root pressure 



(After Mangin.) j^ j^^^^ -^^ ^^^^^^ ^^ transpiration. 



This phenomenon is often to be observed during the summer season in the case 

 of low-growing plants. During the bright warm day transpiration equals, 

 or may be in excess of, root pressure, and the leaves are consequently 

 flaccid. As nightfall comes 

 on the air becomes more 

 moist, and the conditions 

 of light are such also that 

 transpiration is lessened. 

 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 o± 

 the leaves due to the excess ( 

 of root pressure over trans- ' 

 piration. Were it not for 1 

 this provision for the escape 

 of the excess of water raised 

 by root pressure, serious in- 

 jury 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 Pig ^^ 



root pressure and transpira- Guttation of tomato plants after connecting the stems by 

 tion are concerned. means of rubber tubes with the hydrant. 



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

 matoes when grown in poorly lighted and poorly ventilated greenhouses suffer 



