20 



EXPERIMENTAL PLANT PHYSIOLOGY. 



EXPERIMENT 24. 



MEASUREMENT OF ROOT-PRESSURE. 



Cut off the stem of an actively-growing plant of Dahlia, Geranium, 

 Corn, Sunflower, or Grape a short distance above the ground, and 

 fasten tightly to the stump in a perpen- p IG . I5> 



dicular position a long glass tube by 

 means of a short section of rubber tubing. 

 Observe the varying height of the sap 

 in the tube from day to day, noting the 

 temperature and moisture of the air at the 

 same time. (Fig. 15.) 



16. Transpiration. The water taken 

 up by the roots finds its way upward 

 through the stem toward the leaves, 

 where a constant diffusion into the air 

 takes place. The diffusion of water 

 from the leaf or other organs of a 

 plant into the air is designated trans- 

 piration. \ Transpiration takes place 

 under the same physical laws as the 

 evaporation of water from a moist 

 membrane. Barometric pressure, light, 

 temperature, humidity, and move- 

 ments of the air are the most impor- 

 tant conditions affecting the process. Apparatus for demonstra- 

 tion of root-pressure. 



The amount of water actually given off (Detmer.) 

 varies also with certain metabolic processes (see Chapter IV.). 

 A plant may be compared to a tube filled with water, with 

 an expanded upper end closed by a membrane, while the 

 lower end is immersed in water. By evaporation an upward- 

 flowing stream is set in motion. 



EXPERIMENT 25. 



LIFTING-POWER OF THE EVAPORATION OF WATER FROM A MEMBRANE. 



Fill a thistle-tube by placing it in a vessel of water, and while in 

 that position cover the large end by a tightly-stretched membrane. 



