The movement of growth is two thousand times less rapid than the place
of the proverbially slow-footed snail. Taking the average annual growth
in height of a tree to be 5 ft., it will take a tree a thousand years
to cover a distance of a mile. We take a piece of 2 ft. in the course of
half a second, during the interval plant grows through a length of
1,100,000 part of an inch or half the length of a wave of light. For
investigation on the effect of external conditions on growth we have to
measure even a fraction of that excessively small length.
The peasant has eagerly watched the growth of his plants on which his
own life and the world's depend and, even realised something of its
vicissitudes, so the vegetable physiologist has here one of the many
problems of his science. The invention of growth-measuring instruments
has thus been one of his main endeavours. He has hitherto succeeded by
the use of levers with unequal arms to obtain a magnification of about
20 times, and even then it takes many hours for growth to become
perceptible; owing to the practical impossibility of maintaining the
external conditions constant for so many hours, the results of
measurement of growth become vitiated. It is therefore necessary to
produce a magnification so high that growth should become measurable in
less than a minute. The first improvement effected by the lecturer, now
some fourteen years ago, was his Optical Lever, which at once raised the
magnification from 20 to 1000 times, an advance which at the time seemed
to many incredible, but it is at length coming into use in advanced
laboratories in Europe.