136 AUXANOMETEKS. [CH. VI 



We find that a convenient method of measuring the 

 distance between marks is the following. The root is 

 laid on wet blotting-paper to prevent it withering and is 

 supported on a table sliding in a horizontal slot, on which 

 a millimeter scale with a vernier is engraved. A small 

 reading microscope with cross wires is fixed vertically 

 above the root, the table is pushed along the slot and the 

 vernier is read as each mark on the root comes under the 

 cross-wires. We thus read to O'l mm. with fair accuracy. 



Section C. Auxanometers. 



(176) Methods^ 



Instruments for measuring growth are of two kinds : 

 (1) those in which the continuous presence of the 

 observer is necessary and (2) self-recording instruments. 



Of the first class various simple forms may be con- 

 structed. If a cord attached to the summit of a flower- 

 stalk is passed over a pulley (supported vertically above 

 the plant) and attached to a weight, the descent of the 

 weight in a given time equals the elongation of the 

 plant. The descent of the weight may be read in various 

 ways, but in all cases certain sources of error have to be 

 avoided. The cord should be of fine plaited silk, because 

 twisted cords vary in length with the moisture of the air. 

 The weight must only be sufficient to keep the cord 

 thoroughly tight because any serious strain interferes 

 with growth. The cord may be attached by a simple 

 knot or loop, and if there seems any danger of its cutting 

 1 See Sachs' Arbeiten, i. p. 113. Also Text-Book, p. 826. 



