312 VEGETABLE PHYSIOLOGY 



at a minimum, it gradually becomes accelerated, reaches 

 a maximum, and slowly ceases, exactly as did that of the 

 cell which we first considered. By careful examination of 

 a growing root it can be found that the growth is greatest 

 just behind the merismatic region. If a young root be 

 taken and marked out into zones by a series of short lines 

 A B at equal distances apart (fig. 137, A), 



and then allowed to continue its 

 growth, it will be found that the 

 lines remain close together at the apex 

 and for a very short distance from 

 it. Then they become separated by 

 broader spaces (fig. 137, B). Further 

 ^ck still the original intervals be- 

 OF THE EADTCLE. tween the lines will again be found to 



be almost unaltered. The second region corresponds to 

 the part where the cells are undergoing the enlargement 

 described. The total growth of the root is, of course, the 

 sum of the increments of all the zones so marked out. 



The same order of events may be ascertained to take 

 place in the stem, but in this region it is complicated by 

 the occurrence of nodes and internodes. Growth in length 

 is almost confined to the latter, each of which passes 

 through a similar grand period. The growth of the stem 

 is the algebraical sum of the growth of the internodes, 

 many of which may be growing simultaneously and which 

 will be at any particular moment therefore at different 

 parts of their grand period. The region of growth in 

 the stem is, as a rule, much longer than that in the root. 



The growth of the leaf shows a little variation. The 

 apical growth, as a rule, is not very long continued, and 

 the subsequent enlargement of the leaf is due to an inter- 

 calary growing region near the base. This area has the 

 merismatic cells at about its centre, and regions of greatest 

 growth are on both sides of it. This can be traced more 

 easily in the elongated leaves of Monocotyledons than in 

 those of Dicotyledons. 



