174 



PLANT PHYSIOLOGY 



of the plant is made with a cork -borer, and a cut is made with scissors from 

 this to the margin of the piece. This rubber is then placed around the stem, 

 the cut edges near the central hole are stretched to overlap a little, they are 

 sealed together with liquid rubber cement (that used in mending bicycle 

 tires, and everywhere obtainable), and are held clasped until this sets. Then 

 a line of the cement is run to the margin, sealing one edge over the other. 

 When fully set the margin of the rubber is clamped to the shell, all surplus 



*m 



Fig. 44. — Aluminum shells, with rubber roof, for preventing evapora- 

 tion FROM POT AND SOIL. 



material is cut away, and there remains a very neat and perfectly tight roof, 

 readily removable at any time. 



Transpiration Balances. Obviously these should be of a degree of 

 accuracy according with the work to be done. For ordinary demonstra- 

 tion purposes, a balance sensitive to a gram, such as the Harvard Trip-scale 

 (costing about $5.00) or the Springer torsion balance (costing about $14.00), 

 is excellent. Where a more accurate form, necessarily with knife-edge bear- 

 ings, is used, care must be taken to secure very long pan-hangers to allow 

 room for the plant under the beam. The Gerhart balance, No. 3423 (cost- 

 ing about 115 marks), serves fairly well, but better are the special arrange- 

 ments described by Hansen in Flora, 84, 1897, 355, and by Hoehnel, 

 .figured in Burgerstein's "Die Transpiration," 8. Still better is a form 

 without an overhanging beam, permitting the use of a plant of any height, 

 and such is figured by Pfeffer, i, 241. An objection to all these balances, 

 especially when many weighings must be made, is the slowness with which 

 the approximate weight is found by use of weights. Spring balances are 

 ideal in this particular, but are far too inaccurate for any but the crudest 



