CHAPTER IX 



The Primary Wall of Growing Cells 



IT WILL be recalled that while we have defined the primary wall as 

 the envelope surrounding the growing cell, this delicate membrane 

 still remains present in an adult cell as the outer limiting layer of the 

 wall. It is the purpose of the remainder of this book to attempt a 

 description of the structure of the membrane while the cell is still 

 growing, and to try to assess the present bearing of such a description 

 on the processes of growth. We exclude from the membrane the middle 

 lamella which cements two neighbouring cells together and all the layers 

 which are subsequently deposited after growth has ceased. Such a 

 separation between a growing and a non-growing wall is obviously one 

 of considerable importance, associated with a variety of fundamental 

 differences some of which are immediately obvious. Thus, since the 

 primary wall is growing in area but not appreciably in thickness, while 

 the secondary wall is growing in thickness but not at all in area, there 

 must be some quite distinct difference between the two in relation to the 

 protoplasm and, indeed, possibly to the metabolism generally of the 

 cell. It has been said that cells change from a predominantly protein 

 metabolism to a predominantly carbohydrate metabolism just at about 

 the commencement of secondary wall formation. Though this bald 

 statement can hardly stand nowadays without serious modification, for 

 we know that carbohydrates must be manufactured rapidly in growing 

 cells (see Fig. 3), nevertheless the distinction does hold even if not in the 

 extreme sense first visualized. Concomitant with such a differential 

 relationship, it will be a commonplace to those familiar with botanical 

 material that the staining reactions of growing and adult cell walls can 

 be quite different. For these, and for many other reasons, it is impera- 

 tive always to treat the primary and the secondary walls as two different 

 entities, and this view can be no more forcibly expressed than in these 

 studies of structural relationships. It will become progressively clearer 

 that at each and every step of a structural investigation the behaviour 

 of the primary wall is radically different from that of the secondary. 



This is not to say, of course, that the organization of the primary wall 

 involves the embodiment of any new chemical species or the expression 



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