So 



PLANT PHYSIOLOGY. 



moisture, a hygrograph (page 33) is best, though a wet and dry bulb 

 apparatus frequently observed, will do. There are several forms of 

 sunshine-recorder, but for most purposes an approximate record traced 

 by some student for the class upon a thermograph sheet kept beside 

 him for the purpose is sufficient. All of these records should be plotted 

 upon the sheet with the Transpiration curve. 



There are several other methods of investigating transpiration, of 

 which one of the most practicable is the measuring method described 

 by Detmer (page 217). A very simple and ingenious method of demon- 

 strating transpiration and intercellular aeration together is given by 

 Noll in Flora, 86, 386. Very useful for some purposes are potometers, 

 several forms of which are described by Darwin and Acton. A very 

 practicable form is described by MacDougal in the Botanical Gazette, 

 24, 1 10. A modification of this, which I have found very efficient, is 

 shown in Fig. 15. A T-tube is bent into the form shown by the 

 figure and attached by a water-tight joint to a large-bore thermometer- 



FIG. 15. A SIMI-LE POTOMETER. One-third the true size. 



tube, backed by a millimeter scale. The plant, carefully cut off under 

 water, is fixed by a water-tight joint in one arm of the T-tube, great care 

 being necessary to prevent compressing the ducts while the joint is 

 being made. To the other arm is attached a tube with a glass stop- 

 cock, which, however, may be replaced by the simpler arrangement of 

 rubber tubing and compressor shown at the bottom of the figure. The 



