THE PRIMARY WALL OF GROWING CELLS 173 



SO that the cells are nearing the end of their growth period) is of the 

 order of 14% while in cambial cells (which are not vacuolated to any 

 marked extent) it reaches the low value of only 8 %. It is therefore clear 

 why such walls are much less resistant to mechanical disturbance than 

 are the adult cells, and it is immediately possible to attribute the ready 

 dimensional changes undergone by growing cells in part at least to this 

 low proportion of the structural polysaccharide. 



Pectin, hemicelluloses and cellulosans 



It has been known since the pioneer work of Mangin, beginning in 

 1888, that pectic substances are of wide occurrence in primary walls 

 where they occur in high proportion. Normally they are completely 

 insoluble in water and were originally termed "protopectin" or "pec- 

 tose" to express this insolubility. The idea is now rather widely held 

 that pectic substances in these walls occur largely in the form of the 

 calcium-magnesium salts of partially methylated pectic acid, rendered 

 insoluble either by the possession of long chain length or, and more 

 probably, by entanglement, either mechanical or even chemical, with 

 other molecular chains such as those of the cellulose. It has frequently 

 been suggested that the preponderance of pectic compounds in growing 

 cells may be connected with the high oxidation rate which is a feature 

 of growth, the idea being that the enhanced oxidation may be con- 

 nected with the development of — COOH groups in place of — CHgOH. 

 This would not, of course, necessarily imply that glucose, which would 

 otherwise have gone to cellulose, is being side-tracked through an oxida- 

 tion to pectin; the process must be much more complicated than that. 

 The hemicelluloses, including the cellulosans, are commonly even more 

 abundant in primary walls (Table I) and often take the form of xylan. 

 Remembering the relation between cellulose and xylan demonstrated 

 in the much more robust secondary wall, it may well appear that the 

 peculiar reaction of the cellulose here may in some measure be due to 

 heavy contamination with this very similar molecular species. Lignin 

 is normally absent from primary walls, or present in such small amounts 

 as to suggest that it enters the analysis as a contamination from the 

 debris of neighbouring mature tissues. 



Protein 



Perhaps the most striking feature which comes out of any analysis of 

 primary wall material is the frequent presence of protein. It has natur- 

 ally to be remembered that the wall is in close association with the 

 protoplasm within it, and that this involves some difficulty in separation. 

 On this account some authorities have attributed the protein to a 



