CHEMICAL NATURE OF THE CONSTITUENTS 29 



extremely complicated molecules of high molecular weight might be 

 built up. 



The physical properties of Hgnin are also consistent with such a view 

 of its constitution. Thus the high refractive index (1-61) is considered 

 to be in harmony with the proposed structures, and the ultra-violet 

 absorption maximum at 2800 A. is in accord with the absorption 

 spectrum of known aromatic compounds. While the molecular weight 

 of lignin is therefore probably high, the figures recorded in the literature 

 for extracted lignin are rather low, the value depending on the technique 

 of isolation and upon the method of molecular weight determination. 

 Using the ultra-centrifuge Gralen(lO), for instance, has recorded 

 molecular weights as low as 3500, though in other instances the figure 

 turns out to be of the order of 40,000. On the whole, therefore, the 

 evidence suggests that lignin does not form long chain molecules like 

 cellulose and the other polysaccharide derivatives in the wall, and it is 

 understandable why earlier attempts to find a precursor of lignin among, 

 for instance, the pectic substances have led to such dismal failures. 



Staining reactions 



So much, then, for the chemical nature of the material whose physical 

 attributes are to be studied in the following pages. One last word may, 

 perhaps, be said about the common methods of detecting the presence 

 of the various substances concerned since these will be referred to again 

 and again in this book. None of the staining reactions for the incrusting 

 substances can be said to be specific, but if appUed with care and with 

 due recognition of the dangers involved, and particularly if considered 

 in conjunction with specific solubility tests, they can be used with some 

 degree of certainty. 



Taking cellulose again first, the blue coloration with iodine followed 

 by 70 % (by weight) of sulphuric acid is generally accepted as a certain 

 test of its presence.* The absence of a blue coloration is not, however, 

 any guarantee that cellulose is absent, for a chip of wood gives no 

 cellulose reaction although perhaps as much as 60 % of its dry weight 

 is made up of this polysaccharide. Only after hgnin removal is a positive 

 reaction obtained, so that with negative responses to this test due 

 consideration has to be given to the possible masking effects of other 

 substances. Among certain species of the algae a positive test for cellu- 

 lose is obtained only with the greatest difficulty for other reasons which 

 will appear later on. Cellulose can be further characterized among 

 substances liable to be present in cell walls by its ready solubility in 



• There are, however, prominent exceptions which will not be discussed here. 



