1] UPON THE RATE OF GROWTH 373 



of a Cannabis seedling there is a momentary elongation followed 

 for six hours by no growth whatever ; then periods of growthT 

 alternating with periods of retardation occur until, after 

 perhaps 24 hours, the hourly growth is nearly equal to that 

 of the unweighted plant. The least weight which will cause a 

 retardation is different for different plants. In the case of 

 Cannabis and Linum it is below 1.3 grammes, a result which 

 indicates that the weight of the index used in self-recording 

 auxanometers is sufficient to retard the growth of the plant and 

 thus to give an abnormal growth curve. 



The diminution of growth is not the same at all periods of 

 the plant's development. Thus the retarding effect is greatest 

 at the commencement of the grand period of growth, but begins 

 quickly to diminish until at the maximum of growth there is 

 no retarding effect. Then, as the rate of growth decreases, the 

 retardation becomes more evident. The effect of the pull is 

 best seen in comparing the curves of daily fluctuation of growth 

 of the weighted and unweighted plants. The early morning is 

 the period of rapid growth, and at this time the growth of the 

 stretched plant is quite equal to or exceeds that of the un- 





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NIGHT 



FIG. 100. Curves of hourly growth, measured in hundredths of a millimetre, ver- 

 tical scale at the left. The full line gives the course of growth of a Cannabis seed- 

 ling subjected to a pull of 13 grammes. The dotted line gives, for comparison, 

 the course of nearly normal growth, the plant being subjected to a pull of only 

 3 grammes. The numbers at the bottom indicate hours of the day ; the broken 

 line at the bottom indicates night-time, (From HEGLER, '93.) 



weighted one. During the early night, on the contrary, when 

 growth is feeble, the stretched stem grows slowly (Fig. 100). 

 If an etiolated plant is used, the daily growth fluctuations are 

 eliminated and the effect of the larger growth cycle becomes 



