16 



THE PLANT CELL WALL 



dichroism. By taking the utmost precautions for the exclusion of 

 oxygen during cuprammonium extraction, a molecular weight for 

 cellulose of over 20,000,000 has been obtained, corresponding to 

 a degree of polymerization of 100,000 or more, and a molecular 

 length of at least 0.05 mm. 



Source 



Method 



Degree of 

 polymerization 



Cotton 



Ramie 



Hemp 

 Spruce 



The molecular weights of pectic and pectinic acids vary consider- 

 ably with source, measurement technique and isolation method. 

 There is some question still about the intactness of isolated pectins, 

 hence the reported molecular weights may represent a greater 

 or lesser fraction of the native molecule. They may, however, be 

 taken as minimum values. Among the reported molecular 

 weights are: apple pectin, 67,000; apple pectic and pectinic acids, 

 62,000-280,000; lemon pectin, 89,000; beet pectin 62,000; beet 

 pectinic acid, 90,000; and flax pectin 64,000. 



The pectins are characterized by their content of ester methyl 

 groups. The ideal limits for these polymers are unesterified poly- 

 galacturonic acid and the fully esterified molecule which has no 

 acidic properties. The intermediates with varying proportions of 

 free and esterified carboxyl groups represent the pectic and pectinic 

 acids. The maximum methoxyl content for pectins containing 

 twenty-five or more residues is 16.3 per cent. The actual methoxyl 

 contents vary from the low ester range, 3-7 per cent, to high val- 

 ues of 12-15 per cent. As the methoxyl content is lowered, the 

 pectic substances become more sensitive to di- or poly-valent cations. 



The constitutional differences between protopectin and the 

 other pectic substances remain unclear. Empirically, the protopectins 



