IlECOEDiNG AUXAlJOMETfeRg. 



383 



1004. Measurement of growth. In some cases it is very easy 

 to make direct measurements of the amount of increase in vol- 

 ume ; but in general it is necessary to employ some form of appa- 

 ratus by whicli the amount can be more or less exaggerated by 

 a multiplier. 



Several forms of growth-measurers, or auxanometers, have 

 been devised for attaining this end. The simplest consists of 

 a fixed arc of large radius (see Fig. 168), on which a delicate 

 arm moves up or down according to the direction in which a 

 small wheel at the centre, to which the arm is attached, is moved 

 by the action of a thread fastened to the plant. Care must be 

 taken to balance the arm as perfectly as possible, in order to 

 prevent any strain on the plant by the weight of the index. 



This form of apparatus is well adapted to demonstration before 

 a class ; and if a rapidly 

 growing seedling or strong 

 scape is chosen for experi- 

 ment, the movement of the 

 arm through the arc in an 

 hour will be sufficient to be 

 clearly seen at a considera- 

 ble distance. A modifica- 

 tion of the apparatus bj' 

 Professor Bessey reduces 

 its cost to a mei-e trifle. 

 Both the arc and its sup- 

 porting radii are made of 

 strong manila paper ; the 

 wheel is a comrnon spool, 

 and the arm may be a slen- 

 der straight straw. 



1005. Recording Auxano- 

 meters. For the purpose of 

 registering growth, several 

 applications of the chrono- 

 graph have been made. 

 One of the most satisfactorj' 

 of these consists of a slowly 



revolving cylinder covered with smoked paper, upon which a 

 needle, attached to the end of a balanced thread passing over a 



Fig. 169. Registering auxanoraeter. The thread attached to the plant passes over 

 ttie small wheel at x, and is balanced by a weight. The index z is balanced by the 

 weight g ; the thread between theia goes over the wheel r. The cylinder is carried round 

 by the clock-work, which is regulated by the pendulum weight at^. (Pfeffer.) 



169 



