180 PLANT PHYSIOLOGY 



and at the top of its swing is made to touch a wire so arranged that an elec- 

 tric circuit is thus closed; this excites the electromagnet and thus raises 

 the releaser-slide which drops a ball into the scale-pan, at the same 

 moment making a mark on the record paper. The weight of the ball then 

 depresses the scale-pan, breaking the circuit. This operation is repeated 

 thereafter every time the plant has lost a gram of water. The drum revolves 

 once in twenty-four hours, and the record paper, which shows clearly in 

 the figure, is divided into numbered spaces corresponding to the hours; 

 these spaces are subdivided into twelve parts, each therefore representing 

 five minutes, and these in turn can easily be read by estimation to fifths, 

 or one-minute intervals. It is possible, therefore, to -read off from the drum 

 directly the number of minutes it takes the plant to lose one gram of water, 

 data which are readily transformable into other terms. The record paper 

 is divided into seven horizontal spaces, marked by initial letters, one for 

 each day of the week. The pen slides on the bar, which contains seven 

 notches; and each day, when the plant is watered, the clockwork is wound, 

 the weights are returned to the cylinder, and the pen is slipped along the 

 bar one notch. These operations are to be performed between 8 and 9 

 a.m. daily, this hour being duplicated on the paper for this purpose. Each 

 record paper therefore contains a complete record for a week. The mechan- 

 ism is protected by a glass bell jar. In arranging the instument for use, 

 one first places the record paper upon the clock-cylinder, lifting the latter 

 vertically from the clockwork for this purpose. The paper is first wrapped 

 tightly around the cylinder with its bottom margin matching the lower edge 

 of the cylinder; the outer end (that marked with the letters M, T, W, etc.) 

 is then lightly mucilaged on its under face and pressed tightly over the inner 

 edge, and the cylinder is allowed to rest on a table with this part down for 

 a few minutes, until the mucilage has set. Two or three bits of gummed 

 paper are placed over the top of paper and cylinder to prevent any slipping 

 of the former on the latter. The cylinder is then replaced on the clock- 

 work, with approximately the correct time opposite the pen, and is given 

 the exact adjustment for time by means of the central nut beneath the stage 

 of the instrument. The pen is then filled with recording, or chronograph, 

 ink; this may best be accomplished by aid of a pointed glass rod lifting 

 drops from the bottle. If the pen gives too coarse a mark, it can be made 

 finer by filling the pen with packed cotton-wool and merely moistening the 

 wool with the ink. The pen carrier should be so adjusted that after striking 

 the paper it springs back a trifle, and so remains until the opening of the 

 circuit. The transpirograph is now brought so close to the scale-pan con- 

 taining the plant that the outlet tube extends over a small receptacle placed 

 on the pan to catch the weights; and plant and receptacle are then balanced 

 exactly by weights in the usual way. The instrument and balance are then 

 brought into electric circuit with two dry-battery cells, through the two bind- 

 ing screws under the transpirograph stage; and any convenient arrangement 

 is made on the balance such that the rise of the plant-pan to its uppermost 

 position will close the circuit, when the mechanism should operate as above 

 described. A cut-off in the circuit is also desirable, to permit manipulation 



