234 LECTURE XII. 



lignose, C 18 H 26 O U as a basis) (p. 21), we may infer with Sachs 

 that it is a residue of the oxidative decomposition of cellu- 

 lose, carbon dioxide, water, and possibly a highly oxidised 

 acid such as the oxalic, being the other products. This 

 view harmonises with what has been said above (p. 228) with 

 reference to the oxidative decomposition of carbohydrates 

 in the plant, though in the illustrations there given the 

 oxidation is represented as being more complete. 



In view of the facts mentioned above with regard to 

 tannin, we must regard the glucosides as being, most pro- 

 bably, products of the destructive metabolism of protoplasm. 

 They occur normally in the cell-sap, but they can commonly 

 be detected in the cell-wall by which they are gradually 

 absorbed as the cell grows old and loses its protoplasmic and 

 watery contents. 



The more common glucosides are tannin, salicin, phloridzin, coniferin. 

 They are decomposed by ferments according to the following equations. 



1. Tannin, C 3i H^O^ (Schiff), 



(glucose) (digallic acid) 



a. CMH^OM + 2H 2 O = C 6 H 12 O 6 + 2C 14 H 10 O 9 , 



(gallic acid) 



b. C 14 H 10 O 9 + H 2 O = 2C 7 H 6 O 5 . 



2. Salicin, Ci 3 H 18 O 7 , 



(saligenin) 



Q3H 18 7 + H 2 = CeH^Oe + C 7 H 8 2 . 



3. Phloridzin, C^R^Oio, 



(phloretin) 



C^H^Oio + H 2 O = C 6 H 12 O 6 + C 15 H 14 O 5 . 



4. Coniferin, C^H^Os, 



(coniferylic alcohol) 



H 2 O = C 6 H 12 O 6 + C 10 H 12 O 3 . 



It will be seen, from the foregoing decompositions, that 

 the glucosides yield various aromatic substances ; they may 

 be regarded, therefore, as substances intermediate between 

 the carbohydrates and the purely aromatic bodies. Though 

 they are products of destructive metabolism, yet they are 

 still of some use in the economy : by their decomposition 

 energy is evolved, and the plant may avail itself of the pro- 

 duced glucose. 



