826 
MECHANICS OF GROWTH, 
requires a considerable time in order to overcome the nocturnal, darkness a con- 
siderable time to overcome the diurnal condition of growth of the plant. If this 
were not the case, the curve of growth would at once rise abruptly in the evening 
when the room is suddenly darkened, would then continue at the same elevation till 
morning, would fall abruptly when light is again let in, and would then continue at 
the same height till the evening. But this does not correspond to the observed 
phenomena. 
In order to study more closely the changes of growth occasioned by internal causes, 
or the dependence of these changes on external conditions, it is necessary to meiisure 
Fig. 480.— Arc-indicator, or apparatus for measuring- the growth in length of a plant 
during short periods of time. 
the increments in short spaces of time such as an hour or two or three hours. In the 
case of internodes or leaves of large plants which are growing very rapidly, as the flower- 
stems of Jganje or the leaves of Musaceae, this can be done with a certain degree of 
exactness by simple measurement with a measuring-rod. But for the purpose of more 
exact observations it is more convenient to make use of smaller plants which do not 
grow so rapidly, the growth during an hour not amounting to more than a millimetre, or 
even less. In such cases a simple measuring-rod is not sufficiently exact ; and I have 
employed in its place three different methods. In each of them a thin but strong thread 
of silk is fixed to the upper end of the stem or internode of the plant growing in a 
