DYNA]MICS OF THE GROWTH PROCESS 557 



the deposition of additional molecules on the cell wall layers already present 

 {apposition) . Plastic extension of a wall alone would also result in an in- 

 crease in its area, but if this occurs unaccompanied by the incorporation of 

 new material the wall necessarily becomes thinner. 



A second hypothesis holds that active growth of the cell wall is the 

 primary step in cell enlargement. Growth of the wall is believed to result 

 from the intercalation of additional molecules between those already present. 

 Entrance of water into the cell is considered to be a result of the increase 

 in the volume of the cell rather than its cause. Ursprung and Blum's (191 8) 

 finding that while the diffusion pressure deficit of dividing cells is relatively 

 high, due to the appreciable concentration of solutes present, their turgor 

 pressure is low, is considered to be evidence in support of this hypothesis. 

 Continued areal extension of the wall would tend to keep the wall pressure 

 and hence the turgor pressure of the cell at a low value. 



3. The Maturation Phase. — Size differentiation of cells is largely accom- 

 plished during the enlargement phase of growth, but in some types of cells 

 continues during the maturation phase. During the enlargement phase the 

 cells from which certain tirsues develop enlarge or elongate much more than 

 these from which other tissues develop. The cells of various tissues differ, 

 however, not only in spatial dimensions, but also in various structural features 

 mo:,t of which develop during the maturation phase of growth. Maturation 

 of cells begins earlier in scr:e tissues than in others so that the zones of cell 

 enlargement and cell maturation usually overlap for some distance along the 

 axis of a growing stem tip. Elements of the protoxylem and protophloem 

 often can be distinguished in levels of the growing stem tip in which most 

 of the other cells are still in the enlargement stage. Pith cells also usually 

 become fully matured earlier in the ontogeny of a stem tip than many other 



tissues. 



The cells which develop into the pith and certain other tissues do not 

 elongate greatly along the axis of growth, although their elongation in this 

 direction is greater than radially. Others elongate greatly along the axis 

 of growth and only slightly in other directions, becoming fibers, tracheids, 

 vessel elements, sieve tubes, etc. depending in part upon their position m the 

 organ. Maturation of the cell walls ensues at about the time that cell enlarge- 

 ment ceases. During maturation the walls of practically all cells thicken, 

 although usually not uniformly. The walls of many types of cells and tissue 

 elements become pitted, while in others characteristic structural features are 

 developed, the most striking of which are the spiral and other thickenings 

 of the walls of the protoxylem vessels. 



Chemical differentiation of the cell walls also occurs during the matura- 



