GERMINATION, GROWTH, TISSUE TENSION. 153 



limited is much longer (3 or 4 cm. in Phaseolus epicotyl) 

 than in roots (4 to 8 mm. as a rule) . Hence in dealing 

 with stems and flower-stalks it is sufficient to make the 

 marks 5 mm. apart. 



201. Grand Periods of the Growing Zones. The two 



preceding experiments show that in both root and stem the 

 youngest part grows very little, then comes a region of 

 vigorous growth, and farther back there is again little 

 growth. On continuing the observations, we note that 

 growth soon ceases in the older zones, while the maximum 

 is shifted forward to the younger zones, and still later the 

 rate of growth in these zones in turn diminishes. 



That each individual zone passes through a grand period 

 in this way the zones nearest the apex being at the be- 

 ginning of their grand period and those farthest away 

 from it at the end of theirs is strikingly shown as follows. 

 Mark a single transverse line on a Bean root at a point 

 between 2 and 3 mm. from the tip, then carefully mark a 

 second line 1 mm. behind the 

 first, so as to have a zone 

 1 mm. long in the most 

 rapidly elongating region. 

 At the same hour each day 

 measure this zone and record 

 its daily increase in length. 

 At first the rate of growth 

 is slow, but soon it becomes 

 rapid, reaches a maximum 

 about the third or fourth 

 day and maintains this for 

 about three days, then falls 

 off and by about the tenth 

 day ceases altogether. 



202. Growth Measurement 



Instruments. Varicms special Fi - 38. Ganong's Auxograph (Self- 

 instruments have been devised recording Auxanometer). 

 by means of which the growth of 



organs may be magnified and simply demonstrated (auxoscopes), 

 or measured accurately with or without magnification (auxaiio 



