232 



BOTANY 



is the leaf-sheath of the Banana, which shows an elongation of l'l mm., 

 and a Bamboo shoot, an increase in length of 0'6 mm. per minute ; 

 while most other plants, even under favourable circumstances, attain but 

 a small rate of elongation (0'005 mm. and less per minute). 



In order to measure the growth in length of a plant, it is customary to 

 magnify in some way the actual elongation for more convenient observation. This 

 may be effected by means of a microscope, which magnifies the rate of growth 

 correspondingly with the distance grown. With a high magnifying power the 

 growing apex of a Fungus hypha seems to advance across the field of vision as if 

 impelled by an invisible power. For large objects, the most convenient and usual 

 method of determining the rate of growth is by means of an auxanombter. The 

 principle of all auxanometers, however they may differ in construction, is the same, 



Fig. 194. — Simple and self-registering auxanometer. For description see text. 



and is based upon the magnification of the rate of growth by means of a lever with 

 a long and short arm. In Fig. 194, at the left, a simple form of auxanometer is 

 shown. The thread fastened to the top of the plant to be observed is passed over 

 the movable pulley (r), and held taut by the weight (g), which should not be so 

 heavy as to exert any strain on the plant. To the pulley there is attached a 

 slender pointer (Z), which is twenty times as long as the radius of the pulley, and 

 this indicates on the scale (S) the rapidity of the growth, magnified twenty-fold. 

 By a growth in the length of the plant-stem of J- mm., the pointer would accord- 

 ingly register 4 mm. 



Self- registering auxanometers are also used, especially in making extended 

 observations. In Fig. 194, at the right, is shown one of simple construction. The 

 radius of the wheel {£) corresponds to the long arm, and the radius of the small 

 wheel (r) to the short arm of the lever, in the preceding apparatus. Any movement 

 of the wheel, induced by the elongation of the shoot, and the consequent descent of 

 the weight (67), is recorded on the revolving drum (C) by the pointer attached to the 

 weight (Z), which is, in turn, balanced by the counterweight ( IV). The drum is 



