DYNAIVIICS OF THE GROWTH PROCESS 555 



a question arises regarding the mechanism by which water and solutes, and 

 particularly the latter, are transported into the dividing cells. The rate of 

 movement of solutes towards meristems is usually too rapid to be accounted 

 for by simple diffusion. It is possible that such cells possess the capacity of 

 accumulating solutes or at least of accelerating the rate at which they are 

 translocated as a result of metabolic activity. At least part of the transloca- 

 tion of water and solutes through this zone of the stem axis may occur through 

 the sap-filled intercellular spaces. 



Regions of dividing cells are invariably centers of intensive respiratory as 

 well as assimilatory activity and considerable quantities of carbohydrates are 

 oxidized by such cells. Cell for cell the respiratory rate in meristematic regions 

 is higher than in fully matured tissues. Dividing cells undoubtedly utilize 

 energy in many different ways, a number of which have already been listed 

 in Chap. XXIX. 



2. The Cell Enlargement Phase. — Some cell enlargement usually occurs 

 preceding, during, and immediately following cell division, but usually the 

 increase in the size of the cells during this phase of growth is small compared 

 with that occurring subsequently. Enlargement of cells usually occurs in 

 some tissue zones while cell division is still in progress in others. In many 

 stem tips enlargement and even maturation of the pith and vascular tissues 

 may be occurring while the cells from which the other tissues develop are 

 still in the dividing condition. 



While enlargement of cells usually ensues immediately after cell division 

 this phase of growth is not always completed without interruption. In the 

 formation of the buds of woody plants, for example, cell division is followed 

 by only a very limited increase in the size of the cells and the subsequent 

 phases in the growth of the enclosed stem tip are not resumed until the bud 



opens. 



Many of the cells produced by an apical stem meristem enlarge principally 

 in a direction parallel to the axis of the stem, hence this phase of growth 

 is often called cell elongation. Both the continued formation of new cells 

 by division and their subsequent elongation result in projecting the stem tip 

 forward along its own axis, which is one of the most obvious manifestations 

 of apical growth. Cell division is generally restricted to the uppermost inter- 

 nodes, but the zone of cell elongation often extends over a long series of 

 internodes. The rate of elongation becomes progressively slower, however, 

 with increasing distance of the internode from the stem tip. The elongation 

 region back of a stem tip is sometimes as much as lO cm. in length, and in 

 twining plants even longer. 



Increase in the volume of all cells does not occur equally; neither do they 



