CHAPTER XI 



THE AUTOMATIC RECORD OF GROWTH 



THE movement of the leaf of Mimosa is very sudden and 

 conspicuous, while the movement of growth is almost 

 imperceptible. But the large movements of stems, leaves 

 and roots under the action of various forces such as light, 

 warmth and gravity are ultimately due to excessively 

 minute variations in the rate of growth. The discovery 

 of laws relating to the movement of growing organs thus 

 depends on accurate measurement of normal growth and 

 its changes. Apart from theory, the subject is a matter of 

 great practical importance since the world's food supply 

 is so intimately dependent upon vegetative growth. 



The extreme difficulty of the investigation arises from 

 the extraordinary slowness of growth ; of this we may 

 form some idea from the following examples. Taking 

 the annual growth in height of a tree to be five feet, which 

 is a liberal estimate, it would take a thousand years for 

 growth to cover a mile. The slowness of the snail is pro- 

 verbial, but its pace is 2000 times faster than the average 

 movement of growth. Yet one more instance. We take 

 a single step, covering two feet in about half a second ; 

 during this period the plant grows through a length of 

 i ooooo P ar t of an inch, or half the length of a single wave 

 of light. It is evident that some very strongly magnifying 

 arrangement must be employed to observe growth and 

 its changes. The instrument hitherto used in the botanical 

 laboratory the ' auxanometer ' magnifies about twenty 



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