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 
AB at equal distances apart (fig. 137, 4), 
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. 187, 8). Further 
me Pein back still the original intervals be- 
or THE RaDIcLe, 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. 
