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 measure 



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 Jgave 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 



