ELIMINATION 



187 



tions. The shoot selected should have some firmness of texture so as not 

 readily to wilt, should be cut under water so as to prevent clogging of the 

 ducts by air, and should be handled always as gently as possible. While 

 results become immediately visible, these are more accurate if the shoot is 

 first allowed to stand an hour or two in order to adjust internal pressures, 

 etc.; and a few minutes (10-15) should be given under each new condition 

 for adjustment in temperature and the like. While not in use the plant 

 should be kept covered from dry air; and since the shoot steadily deteriorates, 

 all study of it should be brought 

 within a few hours. For comparative 

 records one may either read the dis- 

 tance the index bubble travels in a 

 given time, or else the length of time 

 it requires to travel a given distance. 

 The latter is the better method since 

 it eliminates a possible error from 

 varying diameter of the tube. One 

 can always easily test whether the 

 absorption shown by the record tube 

 correctly represents the transpiration 

 by simultaneously weighing the instru- 

 ment upon a good balance. 



For demonstration or other merely 

 qualitative purposes, the external 

 conditions may be varied by simple 

 devices of shading, covering with a 

 glass case, etc., the rate of movement 

 in the record tube responding sen- 

 sitively in a striking manner. Or, used 

 by individual students, it may be car- 

 ried from place to place where the 

 conditions are different. But for exact 

 work the external conditions must be 

 varied under control, for which pur- 

 pose the shoot should be passed into FIG. 50. SUPPORTED BELL-JAR; Xf 

 a bell jar (compare KOHL'S arrange- 

 ment in BURGERSTEIN, 16, and DETMER, 221). An arrangement for this 

 purpose, supplied among my normal apparatus, is constructed thus (Fig. 50) : 



It consists of three parts. First is the bell jar of standard size and form, 

 whose ground stopper may readily be replaced by one of rubber carrying 

 inlet and outlet tubes. Second is the firm ring-support of iron having a 

 projecting inside rim and three sockets holding metal legs which may be 

 clamped in any desired position. Third are the thick glass plates which 

 rest upon the ring; one of these is perfectly plain, for use when it is desired 

 simply to seal in an entire plant, but the other is split through its diameter 

 and has a hole i cm. in diameter in its middle. In use the stem passes through 

 this hole, the remainder of which is completely filled by split rubber tubing 



