GROWTH. 



359 



measurements. He found that in the course of their growth 

 irregular periods (indicated by the occurrence of maxima) 

 occurred : thus in plants of Gesneria tubiflora the period 

 usually extended over 2 4, and sometimes over 6 8 hours, 

 and in some etiolated plants of Brassica Rapa he detected 

 a fairly regular daily period in the growth of the stems, the 

 maximum occurring once in every 24 hours at approximately 

 the same time for each particular plant. 



Drude studied the growth of the leaves of Victoria regia. 

 In consequence of the great rapidity of growth he was able 

 to make an observation every five minutes ; hence his results 

 exhibit very clearly the frequency and the extent of the spon- 

 taneous irregular variations in the rate of growth. 



The following are examples of Drude's observations : they begin at 

 1 1 p.m. on Aug. 4 and extend to i a.m. on August 5 : the temperature 

 both of the water and of the air varied scarcely at all during the time, 

 and the plant was exposed to candle-light : the measurements refer to 

 the growth of the petiole only, and are in millimetres. 



In our consideration of growth of length, we have hitherto 

 tacitly assumed that all the cells which constitute any given 

 transverse zone of a growing organ are growing with the 

 same rapidity at any given moment. But this is by no means 

 always the case. In organs which grow rapidly the rate of 



