172 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



and will properly be discussed later, but there are some chemical aspects 

 also which should be dealt with now. 



Cellulose 



Cellulose is usually recognized among botanists in virtue of the blue 

 stain developed when the wall is treated with iodine after a preliminary 

 swelling with sulphuric acid and, although there are cases in which both 

 positive and negative reactions can be misleading, it nevertheless 

 remains true that, with suitable safeguards, any variation in the staining 

 can be interpreted in terms of the presence, modification, or absence of 

 cellulose. Now the walls of many growing cells are found to stain in 

 aqueous iodine alone. This was first observed by Ziegenspeck, who 

 deduced therefrom the presence of a substance other than cellulose, 

 which he called amyloid on account of the resemblance to the famihar 

 starch reaction. Such walls, however, invariably give the normal 

 cellulose stain as they grow older, and Hopman has observed inter- 

 mediate conditions. Nevertheless this absence of a typical staining 

 reaction in young cells, coupled with the difficulty of demonstrating the 

 presence of cellulose by X-rays (see p. 174) led some workers to the 

 suspicion that many, if not all, walls of very young cells were non- 

 cellulosic. We shall see later that such an attitude is no longer tenable 

 in the light of more recent X-ray investigations. It seems now highly 

 probable that cellulose is present from the very beginning, but that the 

 features by which it is normally recognized are masked by the particular 

 conditions in which it grows. In general, it is necessary only to remove 

 some non-cellulosic substance (often of a wax-like nature) in order for 

 the staining reaction to change from negative to positive, and this differs 

 from the condition in many secondary walls only in the smaller response 

 of the smaller amount of cellulose. It seems now, for instance, rather 

 certain that the amyloid of Ziegenspeck corresponds to a complex of 

 cellulose chains only slightly different {e.g. shorter, more dispersed or 

 in some different relation to incrusting substances) from that found in 

 secondary walls. 



The content of cellulose in primary is frequently much lower than it 

 is in secondary walls (Table I) when expressed as a weight percentage 

 of the total dry weight. Normally the primary wall is often swollen 

 with water to a degree much greater than that reached by any secondary 

 wall, with the exception of collenchyma cells, and this therefore implies 

 a still lower volume percentage of cellulose in the fresh growing walls. 

 It can be calculated that the volume percentage of cellulose in the 

 parenchyma cells of oat coleoptiles (where vacuolation has commenced 



