318 VEGETABLE PHYSIOLOGY 
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 at equal distances 
apart (fig. 187, a), and then allowed 
es : 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, 8). Farther back still the 
Seek et cost ot original intervals between the lines 
bic aien Rapicts, will again be found to be almost 
unaltered. The second region corre- 
sponds to the part where the cells are undergoing the enlarge- 
ment 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 ig, 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 Monocotyledong than in 
those of Dicotyledons, 
