LECTURE XV. 



Another point worthy of note is that the duration of 

 growth and the mean rate of growth is not necessarily the 

 same for all internodes. It has been observed in many cases 

 that the internodes first formed in a growing-season are 

 shorter than those formed somewhat later, and that those 

 which are formed towards the end of the season are again 

 short. These differences in length are the expression of 

 differences in the energy of growth of the internodes ; the 

 longer the internode, the greater its energy of growth : and 

 since the length of the internode is the product of the mean 

 rate of growth multiplied by the units of time, differences in 

 the energy of growth find their expression either in greater 

 or less rapidity of growth, or in longer or shorter grand 

 periods. 



The grand period of growth may be conveniently repre- 

 sented by curves constructed with units of time as abscissae 

 and units of increment in length as ordinates. If the curve 

 for any organ be constructed from measurements made at long 

 intervals, say of 24 hours, the outline of the curve will be 

 tolerably even : but if it be constructed from measurements 

 made at short intervals, the outline of the curve will be found 

 to be very irregular, and the irregularity will be the greater 

 the shorter the intervals at which the measurements are 

 made. These irregularities are to be ascribed, to some ex- 

 tent, to variations in the external conditions, to variations of 

 temperature, in intensity of illumination, etc. ; these we shall 

 subsequently consider in detail under the head of the Daily 

 Periodicity of Growth. But many of them cannot be thus 

 accounted for, and these must be regarded as due to varia- 

 tions in the rate of growth which are dependent upon con- 

 ditions inherent in the organ. Sachs was the first to draw 

 attention to these irregularities in the curve of the grand 

 period, and he termed them " stossweise Aenderungen." 



In illustration of these irregular spontaneous variations 

 the following instances may be mentioned. 



Baranetzky, in his researches on the growth in length of 

 stems, kept various plants for some days in darkness at as 

 nearly as possible a constant temperature,, and made hourly 



