GROWTH 317 
In the merismatic tissue of a growing point there is generally 
least resistance on the side of the free apex of the organ, 
and hence an increased protrusion of the latter results. 
Whatever may be the distribution of such pressure the 
growth of the cell will be greatest in the line of least resist- 
ance. If any internal cause should give rise to differences 
in the uniformity of hydrostatic pressure in all directions, 
the growth will be most extensive in the line of the greatest. 
In the second place the extensibility of the cell-wall may be 
locally modified by the protoplasm, so that a uniform 
internal hydrostatic pressure may affect one part more than 
another, and the growth consequently become irregular, 
giving rise in many cases to cells of curious form. 
If we consider the behaviour of a growing organ in the 
light of these facts, we shall see that, like the cell, it must 
show a grand period of growth. If we take the case of a 
root, in which the changes can be traced most easily on 
account of the simplicity of its structure, we find that just 
behind the apex the cells are all in active division. Growth 
is small and consists mainly in an increase of the quantity 
of protoplasm, for the cells divide again as soon as they 
have reached a certain size. As new cells are continually 
formed in the merismatic mass, those which are farthest 
from the apex gradually cease to divide and a different 
process of growth takes place in them, which is associated 
more particularly with the formation of the vacuoles and 
consequently with the establishment of considerable hydro- 
static pressure, thus causing the bulk of the cells to be greatly 
enlarged, as we have described. Hence it is here that the 
actual extension in length of the root goes on, and the 
cells reach the maximum point of the grand period. They 
then gradually loge the power of growth, the oldest ones or 
those farthest from the apex parting with it first, and they 
pass slowly over into the condition of the permanent tissue 
(fig. 138). In this way each zone of the root which may be 
distinguished goes through a grand period of growth. At 
first when the cells are merismatic, growth is at a minimum, 
