42 LECTURE III. 



is diluted with water and boiled, glucose is formed. Cellulose is found 

 almost exclusively in the vegetable kingdom. In the animal world it has 

 only been identified with certainty in the shells of the tunicata. 1 In the 

 cell walls of plants there are found not only sugars of the cellulose group, 

 but other complicated carbohydrates as well, which, on being subjected to 

 hydrolysis, sometimes yield glucose, sometimes no glucose at all, besides 

 other sugars (arabinose, xylose, etc.). These substances have been 

 designated by Schulze as hemicelluloses. 2 In building up the cell walls, 

 furthermore, the pentosans, which yield only pentoses on hydrolysis, also 

 take an active part. 



As is well known, the cell walls undergo changes with age which at first 

 are manifest externally only by greater rigidity. We speak of lignification. 

 This process has been made the subject of much careful investigation 

 without ever being clearly explained. Erdmann 3 assumes that " wood " 

 is formed from cellulose by its combination with other substances which 

 are perhaps of an aromatic character. 4 



Exuding from the various tissue-complexes (medullary-, wood-, and 

 bark-parenchyma) of the cell membranes come the different gums. They 

 are very widely distributed in nature, and, by breaking them down with 

 dilute acids, usually galactose and arabinose are formed. Naturally this 

 group cannot be regarded as homogeneous. Especially well known are 

 gum-arabic and cherry-gum. To this class belongs agar-agar (obtained 

 from East- Asiatic algffi) , which has become important as a culture medium 

 for bacteria. Again, the common vegetable mucilages are included, being 

 different from the gums only by their insolubility, or difficult solubility, 

 in water. 



We now come to those members of the carbohydrate group which the 

 animal and vegetable organisms temporarily withdraw from the general 

 metabolism in order to be able to make use of them at any time by trans- 

 forming them back into simple sugars. We have, in cane-sugar, already 

 met with such a reserve-substance for plants. At least an equally impor- 

 tant part is taken by the starches 5 (amylum) which are found in the 

 seeds, roots, bulbs, tubers, pith of trees in winter (especially in vegetation 

 robbed of their leaves at this season of the year), etc. The amount of 

 starch present in some of these stores may amount to even eighty per cent 

 of the dry substance. Amylum occurs in the form of stratified granules, 



1 C. Schmidt: J. pr. Chem. 38, 433 (1846). Franchimont: Ber. 12, 1938 (1879). 

 Winterstein: Ber. 26, 362 (1893). 



2 Schulze, Steiger, and Maxwell: Z. physiol. Chem. 14, 227 (1890). Schulze: ibid. 

 16, 387 (1892); 19, 38 (1894); Ber. 22, 1192 (1889), and 24, 2271 (1891). 



3 J. Erdmann: Ann. Suppl. 5, 223 (1867). 



4 Cf. Viktor Grafe: Monatsh. 25, 987 (1904). 



6 Cf. Brown and Heron, Ann. 199, 165 (1879). 



