GROWTH 311 
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 resistance. 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 
will 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 lose 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. 183). 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 
