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. 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). Farther back still the 

 FIG. is:. GERMINATING original intervals between the lines 



BEAN, SHOWING GROWTH . . 



OF THE RADICLE. 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 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. 



