174 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



contamination from the cytoplasm and have consequently denied its 

 presence in the wall proper. Thus to quote only two cases, Wood (see 

 references in 54) has denied that proteins may be present in walls to 

 extents more than 0-001 %, and Thimann and Bonner (54), while giving 

 a figure of 12% in oat coleoptiles, allow that some of the protein may 

 be present as a contaminant adsorbed from the protoplasm. Chemical 

 analyses are therefore, by their very nature, suspect. 



Staining reactions are usually more convincing though here again 

 there are sceptics. Many observers, ranging from Krabbe in 1894 

 through Tupper-Carey and Priestley in 1923 to others up to the latest 

 times, have claimed a positive staining reaction for proteins in young 

 cell walls and there have been few dissentient voices in the present 

 century. We shall see later that there are other, and quite undeniable, 

 grounds for suspecting that proteins are present and that they may have 

 a very pronounced effect indeed on the features involved in the increase 

 of wall area associated with the growth process. 



Other substances 



Among the other substances which have from time to time been 

 reported as present in primary walls, and of importance to their 

 identification or their growth or both, we may perhaps notice the 

 frequent references to substances of a wax-like character. A typical 

 example of the effect of such substances is furnished by cotton hairs. 

 Young cotton hairs (less than five days old) yield an X-ray diagram in 

 which the characteristic arcs of cellulose are not recorded with an 

 intensity sufficiently above the background level to be observed. The 

 rather diffuse diagram which is given by these hairs disappears, however, 

 if the hairs are treated with a wax solvent, and the characteristic diagram 

 of cellulose then appears. It seems therefore reasonable to conclude 

 that in the untreated hairs the cellulose diagram was masked by the 

 overpowering wax diagram, but whether or not other complications 

 are involved it is difficult to say. 



On rather similar fines, the presence of phosphatides in primary 

 walls has been claimed by Hansteen Cranner and occasional other 

 workers and, though it has not perhaps yet been substantiated on a 

 sufficiently wide range of materials and conditions, the presence of such 

 substances does seem probable. It is known that the outer layers of 

 the protoplasm do contain these substances and, now that we reahze 

 the close association between these outer layers and the wall itself, there 

 seems every reason to doubt whether any such substance can occur in 

 the one place and not in the other. 



