CELL WALL DYNAMICS 53 



cell undergoes extension. This picture may well apply to the ideal 

 tubular cell, but the existence of the conspicious radial enlargement 

 which has been discussed requires a modified interpretation. The 

 low wall polysaccharide content of young, mitotically active cells 

 suggests a more open, less coherent wall than was realized but 

 a few years ago. The time relations in wall synthesis and the radial- 

 longitudinal transition support the notion that the ability to 

 elongate depends upon formation of a more complete wall. The 

 changeover in the enlargement process is in part a consequence 

 of the directional weakness typical of tubular texture. Thus there 

 must be an intermediate stage after division and prior to elongation 

 in which the tubular character appears. 



The process of surface growth may well be based upon the 

 synthesis of microfibrils at points over the cell surface where the 

 wall comes into intimate contact with cytoplasm. In contrast, there 

 is an older concept of tip growth which is at variance with recent 

 studies on radioactive glucose incorporation. It has also been 

 suggested that the pit fields of the wall serve as centers of cellulose 

 synthesis, but the recent concept of multinet growth seems to be 

 of considerable value in relating wall synthesis with cell extension. 

 By this concept, transversely oriented (optically negative) fibrils 

 are deposited continuously as a loosely coherent layer on the inner 

 surface of the wall during elongation. During extension growth, 

 disorientation of the microfibrils causes them to assume a net-like 

 form. The continuously produced transverse inner layer would 

 tend to maintain its pattern, whereas external, older layers would 

 exhibit progressively greater disorientation. Arena coleoptiles, 

 cotton seed hairs, and other cells provide strong support for the 

 existence of multinet growth. 



Regulation of Cell Wall Formation 



The synthesis of various cell wall components under the in- 

 fluence of 3-indoleacetic acid or other agents has been studied 

 in several tissues. In A vena coleoptiles, for example, the incorpora- 

 tion of acetate into pectates, polyuronides and cellulose is depres- 

 sed slightly, whereas the incorporation of sucrose into protopectin 

 and cellulose is enhanced slightly by 1.8 X 10- 5 M IAA. This auxin 



