322 



PLANT PHYSIOLOGY 



rates are exceptional, as average growth is about 0.005 mm. per 

 minute, but during the grand period of growth cells commonly 

 increase to 100 times their original size in a few hours, and in the 

 Characese they are reported to increase to 2,000 times their orig- 

 inal size during the elongative period. 



Methods of Measuring. — The rate of growth of plant parts 

 may be measured either directly or indirectly. In the direct 

 method, the size of an organ is measured at known intervals, the 



increase noted, and 

 the rate computed. 

 The growth may be 

 actually observed by 

 the use of a horizontal 

 microscope. In this 

 method, the micro- 

 scope is focused on a 

 marked region and the 

 time noted for this 

 region to move a cer- 

 tain distance measur- 

 able on the scale of 

 the microscope. The 

 movement will be due 

 to the growth of the 

 parts below and, for 

 this reason, the tip of 

 the plant is the most 

 convenient region 

 upon which to focus. 



As an indirect 

 method of measuring 

 growth, an instrument called an auxanometer (growth-measurer) 

 is used (Fig. 21). As the plant grows, the wheel rotates and the 

 marker attached to the margin records on the revolving drum the 

 rate of growth, which is magnified by the ratio of the circumfer- 

 ence of the rim of the wheel to the circumference of the center, to 

 which the thread from the plant leads. Various types of auxanom- 

 eters and auxographs have been devised, but one of the most 

 convenient for simple demonstrations is the one devised by Ganong 

 as here figured. 



Fig. 21. — The Ganong auxanometer. For ex- 

 planation of operation see the text. (From Bausch 

 and Lomb Optical Co.) 



