TRANSPIRE TION. 8 1 



Avhole apparatus, including the vial at the right-hand end, is filled with 

 water (boiled to free it from air), and as the plant transpires, a bubble 

 of air, allowed to enter the tube through the vial end, moves along 

 the capillary tube. Its rate may be exactly determined by use of the 

 scale and metronome. When it has nearly reached the end of the 

 tube, the stop cock is opened and the weight of the water in the vertical 

 tube forces the bubble back to the other end of the capillary tube. This 

 apparatus is particularly adapted to studies upon the influence of dif- 

 ferent conditions upon transpiration, since it may so readily be moved 

 about and the plant can be placed in closed receivers, etc. The method 

 assumes that the rate of transpiration and absorption are equal, which 

 may not always be the case, and with me the potometers never work 

 very evenly.* In using it, it is better to employ some stiff-leaved plant, 

 as more delicate ones are apt to wilt. Plants with their own roots, 

 raised by water-culture, should give good results. As arranged above, 

 only relative rates of transpiration are determined, but these may be 

 made definite by use of a small tube of known diameter in place of the 

 capillary tube. (See Note 5 of the Addenda.) 



41. How is the rate of Transpiration affected by external at- 

 mospheric conditions, i.e., by variations in heat, light, moisture? 



Answer by Experiment 20. 



EXPERIMENT 20. To test this, a plant arranged as for Experiment 

 19 may be used, and the conditions varied thus {or in some better 

 way invented by yourselves) : more moisture by putting a large bell- 

 iar over the plant ; less light by a dark screen (use only in sunlight) ; 

 lower temperature by opening the ventilators. The plant should be 

 kept for at least two hours under the new condition, after obtaining 

 for four hours a record for the normal. Every precaution should be 

 used to change only one condition. 



Although this line of work is of very great physiological and ecological 

 importance, it is very difficult to carry it out satisfactorily w r ith simple 

 appliances. It is difficult to keep the external conditions constant for a 

 length of time great enough to allow of good records by weighings. 

 Here potometers, despite their shortcomings, afford fair results, as they 

 give ample records within a few minutes. 



Observation shows that in nature it is often the case that 

 the temperature of the air and of the soil to which a plant is 



* So irregularly does the potometer sometimes work that one concludes the fault 

 must be in it and not in the plant. I have made, however, a very careful series of 

 measurements with the instrument shown in Fig. 15, using instead of a plant one of 

 the diffusion-shells (of Experiment 6) filled with water and exposed to evaporation 

 under various conditions. Thus used, the apparatus worked with the greatest even- 

 ness, showing that the fluctuations are in the plant and not in the potometer. 



