Ch. IV, 7] 



GROWTH OF PLANTS 



155 





so slowly as to need the microscope (specially arranged hor- 

 izontally for the purpose), or else some other magnifying 



)graph in operation 



The thread from the tip of the plant passes over the smaller of two 

 united wheels, and is kept just taut by the weight of the pen, which 

 hangs on the end of the thread passing over the larger wheel. The pen 

 Lgainst the cylinder, which is turned once an hour by a clockwork 

 in the closed case. The growth spiral marked by the pen is exaggerated 

 in the cut. 



device, to make it apparent. A very effective instrument 

 for this purpose, called an auxograph (meaning "growth 

 writer"), is shown by our picture (Fig. 108). The arrange- 



