THE PRIMARY WALL OF GROWING CELLS 175 



The general chemical picture of the primary wall is therefore one of 

 considerable complexity. We certainly have present the polysaccharide 

 cellulose, together with the other sugar derivatives normally found in 

 association with it. In addition to these, however, there is almost 

 certainly present a percentage, even if small, of protein and probably 

 also of phosphatides. This leaves us with the vague suspicion that the 

 primary wall may not after all be a passive coat around the protoplasm 

 such as we have come to regard the secondary wall. We shall find, in 

 fact, as we proceed to the investigation of the structural features in 

 growing walls, that we have progressively more and more reason to 

 suppose that the wall is, on the contrary, taking an active part in the 

 increase in its own dimensions. Here, in fact, in the chemical analysis 

 we have the first suggestion that the wall and the protoplasm at this 

 stage constitute, if not one indissoluble whole, at least two interpene- 

 trating complexes. 



The X-ray diagram of primary walls 



The earlier attempts to elucidate the structure of growing cell walls 

 naturally made use of the less exigent conditions associated with 

 observations under the polarizing microscope. In view, however, of the 

 uncertainty in the interpretation of unsupported observations of this 

 kind, and particularly since, with about only one exception, whole cells 

 or even whole tissues were observed instead of the necessary single walls 

 (see p. 116), it is preferable here to begin our analysis with the later, 

 more rigorous interpretation of the X-ray diagram. Rather detailed 

 studies have now been made both of the cambium of conifer trees and 

 of growing oat coleoptiles, but since the results of the two investigations 

 tally in all material points, only the former will be described here. 



It is clear from what has been said that the first point to ascertain is 

 whether or not the X-ray diagram which can readily be obtained under 

 suitable conditions from dried tissue has any bearing upon the structure 

 present in the fresh tissue. This is all the more essential in view of the 

 recent complete denial of such a connection in cotton hairs. 



Crystallinity in primary walls 



Growth responses such as those elicited by small quantities of growth 

 substances have frequently led to the speculation that the cellulose in 

 the primary wall may not be associated into crystalline lattices as it is 

 in secondary wall, and this seemed at first sight to have received strong 

 support from the work of Berkeley and Kerr (55) on cotton hairs. They 

 found that young cotton hairs photographed fresh from the boll, 

 without any intermediate drying, gave no indication at all of the expected 



