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- 

 sembryanthemun 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 

 TI.EGHEM and DOULIOT. 



