428 BOTANICAL GAZETTE [December 



little of which can be seen in fig. 3. This peg is between the yoke 

 and the base of the U-piece. Another piece of "tin" (r) is hinged 

 upon / by a pin, and has fastened to it by fine wire a rubber pad {p) . 

 Another rubber pad (q) is fastened to the yoke. These pads are 

 pieces of 4 mm. rubber tubing (even smaller can be used for some 

 purposes), and each has a groove cut a little more than halfway 

 around it before it is secured in place. A slot in r fits over the 

 screw (s') when ;' is closed, and then the grooves on the two pads 

 come together and make a place for holding the stem. A peg 

 through the screw (s') keeps this clamp closed. By adjusting the 

 screws (s and s') the correct amount of pressure required to hold 

 the stem from slipping without interfering with its growth is 

 obtained. This is easily determined by a few trials. The entire 

 frame weighs about 20 gm. The clamp for holding the lower part 

 of the stem used is constructed in the same way. 



In order to keep tally on the number of complete revolutions 

 which the stem makes in twisting, a black thread (the fine silk 

 thread being white) is attached to the upper part of the bow of the 

 frame, and, allowing plenty of slack, is then carried over a hook 

 near the wheel (W). The number of times this is coiled (very 

 loosely) about the silk gives the amount of twist roughly, while it 

 is given exactly by means of the mirrors, telescope, and semi- 

 circular scale. For the sake of clearness this black thread also 

 has been omitted from the figure. 



One other device which, although imperfect, was found useful 

 in studying intermediate points in the part of the stem used is here 

 described and is illustrated in figs. 4 and 4a. I call it a "mirror 

 clamp." It consists of two symmetrical halves (a and b) cut out 

 of aluminum and bent into shape as shown. These halves are 

 hinged together at one edge, and when closed form essentially a 

 cube without a bottom. A mirror of quite thin glass, about i cm. 

 square, is held in place on each side of the cube with bits of adhesive 

 plaster. The clamp is opened by means of holders {c and d) on 

 the two halves, and it is kept closed by means of the steel spring {e). 

 On the top of each half there are two projections (h, k and m, n). 

 Fig. 4a shows with enlargement how a piece of stout thread is 

 placed on one pair of these. It is not tied, but the ends are twisted 



