746 . MECHANICAL LAWS OF GROWTH. 



requires a considerable time in order to overcome the nocturnal, darkness a con- 

 siderable time to overcome the diurnal condition 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, fall 

 abruptly when light is again let in, and 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. 450. — Arc-indicator, or apparatus for measuring the development of an internode of a growing 

 plant a 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 Agave 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 

 pot, the thread passing vertically over an easily moveable pulley and moving an index 

 fixed to the free end of the thread or to the pulley. 



