GROWTH UNDER CONSTANT EXTERNAL CONDITIONS. 739 



place in each period of twenty-four hours in a piece originally i mm. long situated 

 immediately above the piinctiim vegetationis : — 



In the same way I found that a piece at first 3-5 mm. long of the first inter- 

 node of Phaseolus muUiflorus beneath the first pair of foliage-leaves, with a daily 

 variation of temperature between 1275° and 1375° C, showed the following in- 

 crease : — 



Since every organ that is growing in length consists of zones of different ages, 

 which are produced in succession from the primary meristem of the pmtctum vegeta- 

 tionis (or of an intercalary vegetative zone), the successive zones of an internode 

 or a root indicated by ink-marks must show different increments of growth in 

 equal times. While the zone nearest i\iQ puncium vegetationis is beginning to grow, 

 the next one has already entered on a later phase of its grand period, while one 

 at a greater distance would have attained the maximum of its rapidity of growth, 

 and a still further one would have ceased to grow. In other words, a number of 

 zones below the cell-producing puiictum vegetationis are in the ascending phase, 

 while those lying further backwards are in the descending phase of their grand 

 period ; or again, each zone is in a later phase of its period of growth the greater its 

 distance from the pwicium vegetationis. If the successive zones of a growing organ 

 are indicated by the figures I, II, III, &c., and the increments of growth observed 

 at the same time in each of them by Ij, Ij, I3, &c. ; then we have the following 

 relationship : — 



I II III IV V VI VII VIII 

 \ < I, < I3 < I, > I5 > Ig > I, > zero. 



There is therefore in the organ a region of maximum rapidity of growth. Thus, 

 for example, 14'ound in the first internode oiPhaseolus multijiorus^ which was divided 

 into twelve zones, each 3-5 mm. long, in the first forty hours: — 



3 B 2 



