420 
PHYSIOLOGY 
this endogenous fashion upon roots, and likewise on old shoots or leaves. 
They commonly owe their origin to some external stimulus (see p. 428). 
Many of the growing points that are formed regularly (exogenously) 
on the shoot do not develop, for one reason or another. They may then 
be overgrown completely in woody plants, and so lie dormant for years, 
to be called into activity when some accident has 
checked the growth of others, formerly more favor- 
ably situated. Not every shoot, then, that appears 
to come from the interior is really endogenous in 
origin. 
Phase of enlargement. As cells newly formed 
in the meristem grow older, they enter gradually 
upon the second phase of development. This is 
characterized by enlargement, oftentimes so great 
and so rapid as to be very remarkable. In this 
period the volume of the cell not infrequently in- 
creases a thousandfold or more, though ordinarily 
much less. Of course this involves rapid growth 
of the cell wall in area, and if the cytoplasm were 
relatively as abundant as in the earliest stage, it 
would require the formation of a large mass of 
costly material. But while the cytoplasm does 
actually increase considerably, much the greater 
part of the cell is occupied by the water which en- 
ters it. Hence an indispensable condition for growth is an adequate supply 
of water; and the dwarfing which results from a deficiency of water is 
partly a direct consequence of the non-distension of the cells in this stage. 
The water enters the protoplasm, doubtless as a result of the formation of 
substances having a high osmotic pressure. It enlarges the minute vacu- 
oles everywhere through the cytoplasm, until some become so distended 
as to merge, forming fewer but larger ones. This process continues until 
in the center a few large vacuoles, or often only one, occupy the greater 
part of the space, while the major portion of the cytoplasm lies next 
the cell wall as a relatively thin layer, containing the nucleus, plastids, 
and other inclusions (see diagram, fig. 619). It will be apparent that 
since this many-fold enlargement is attained so largely at the expense of 
water, plant growth is relatively economical. 
Unequal enlargement. The young cell has its three dimensions 
nearly equal. Enlargement takes place in all dimensions, but to different 
FIG. 667. Endoge- 
nous origin of a lateral 
root (r~) of ice plant (M e- 
sembryantkemun crystal- 
linum) : c, primary cor- 
tex, and e, endodermis, 
ruptured by young root; 
p, pericycle, from which 
it arises; x, primary xy- 
lem element. After VAN 
TIEGHEM and DOULIOT. 
