CH. VI] AUXANOMETERS. 149 



SECTION C. Auxanometers. 



(176) Method^. 



Instruments for measuring growth are of two kinds : 

 (1) those in which the continuous presence of the ob- 

 server 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. If cord is employed, it should be of fine plaited 

 silk, because twisted cords vary greatly in length with the 

 moisture of the air. The alteration of lengths of plaited 

 cord may, as Sachs points out, be to a larger extent 

 prevented by oiling or waxing. It is however in all ways 

 better to use fine flexible wire. The weight must onlv be 



o / 



sufficient to keep the cord or wire 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 the tissues the stem 

 may be protected by a strip of gummed paper wrapped 

 round it before the cord is attached. If wire is used, it 

 should be hooked to a piece of soft cord tied round the 

 plant. Any shrinking or swelling of the earth in the pot 

 will obviously introduce errors. These can only be avoided 



1 Sachs, in his Arbeiten, i. p. 113, and in his Text-book, Ed. n. p. 826. 

 See also Baranetzky, M&n. Acad. St Petersbourg, xxvn. 1879. 



