Ways in Which Plants Increase in Size 



329 



turn the wheels and keep the threads taut; and therefore, as the 

 plant grows, the pen descends, making its mark upon the paper. 

 The descent of the pen, however, is obviously faster than the 

 growth of the plant in just the proportion that the greater wheel is 

 larger than the smaller, this arrangement of the wheels being 



Fig. 121. — An auxograph, or recording growth measurer, in action. Its construction is ex- 

 plained in the text. Unfortunately the record, in the form of a spiral line on the 

 cylinder, does not show in the picture. 



adopted in order to space out the growth record enough for clear 

 visibility. The cylinder, however, is revolved continuously by 

 clockwork, making a complete turn once every hour; and there- 

 fore the descending pen traces not a straight, but a spiral, line, 

 which every hour crosses a vertical line ruled on the paper, mark- 



