474 "MANUAL OF BOTANY 



slowly, increases to a maximum, and then becomes gradually 

 slower till it stops. The time occupied by these varying rates 

 of growth is generally spoken of as the grand period of growth. 



Changes in the shapes of cells arising during growth depend 

 upon two factors. The direction of hydrostatic pressure and the 

 power, of the cell to respond to it may not continue the same in 

 aU directions, and, consequently, the growth of the cell in the 

 direction of greater pressure or least resistance will be greater 

 than elsewhere. The extensibility of the cell-wall may also be 

 locally modified by the protoplasm, so that the growth becomes 

 irregular, and cells of curious form may result. 



If we consider the behaviour of a growing organ in the light 

 of these facts, we shaU see that it, like the cell, must show a 

 grand period of growth. If we take the case of a root, in which 

 the changes can be traced most easily from the simplicity of its 

 structure, we find that just behind the apex the cells are all in 

 active division. Growth is small, for the cells divide again as 

 soon as they have reached a certain size. As new cells are 

 continually formed in front of the merismatic mass, those 

 behind gradually cease to divide, and the process of growth 

 proper takes place. Here the actual extension in length of the 

 root goes on, and the cells are at the maximum point of their 

 grand period. They then gradually lose the power of growth, 

 the oldest ones, or those furthest from the apex, parting with it 

 first, and they pass over into the condition of the permanent 

 tissue. Thus every portion of the root goes through a grand 

 period of growth ; at first, when the cells are merismatic, growth 

 is 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 with 

 a series of dots at equal distances apart, and then allowed to 

 continue its growth, it will be found that the dots remain close 

 together at the apex and for a very short distance from it. Then 

 they become separated by broader spaces. Further back stiU 

 the original intervals between the dots are again unaltered. The 

 second region corresponds to the part where the cells are under- 

 going the enlargement described. 



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 is confined to the latter, 

 each of which passes through a similar grand period. The 



