480 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



metabolism of fats is uncertain ', for during germination the amount present 

 undergoes irregular fluctuations. The existence of cholin in plants is probably 

 connected with the formation and disintegration of lecithin 8 . 



Cholesterin 3 appears to be commonly present. It is an alcohol, and forms 

 ester-like compounds with fatty acids such as the fats of hair and wool, animal 

 products which are probably also produced by plants. 



SECTION 83. The Composition of the Cell-wall. 



The cell-wall, which forms a protective covering for the protoplasts 

 and which subserves other functions as well, is not always composed of the 

 same constituents, for recent researches have shown that it contains one or 

 more anhydric compounds of different carbohydrates with which other 

 substances may be united, so that in this way an endless variety of 

 modifications becomes possible. As a matter of fact mere microscopical 

 examination suffices to detect many differences in the properties of different 

 cell-walls, or even between the different layers of the same wall, and it is by 

 means of the changes involved in suberization, cuticularization, gelatiniza- 

 tion, &c. that cell-walls become specially adapted for different purposes. 



The cell-wall is a morphological term for the protective membrane 

 which encloses the protoplast, and it may differ widely in structure and in 

 chemical composition in different cases 4 . Every cell-wall has its own special 

 chemical composition, and according to the plant examined, glucosans 

 (dextrosans), galactans, arabans, and also gluco-mannans, galactoso-pentans, 

 &c. may form its constituents 5 . 



A typical cellulose membrane when subjected to hydrolytic decomposi- 

 tion yields for the most part only dextrose, but in other cases mannose and 

 other hcxoses as well. Hence it is best to use the term ' cellulose ' as 

 referring not to one but to a group of substances. Schulze terms those 

 celluloses, which are attacked and dissolved with relative ease by acids 

 and alkalies, ' hemicelluloses.' These yield in addition to dextrose other 

 forms of sugar, and though commonly used as reserve-cellulose, they some- 

 times take part in the formation of permanent cell-walls. The hemi- 

 celluloses form all variety of intermediate stages between typical celluloses 



1 Literature : E. Schulze, Versuchsst., 1894, Bd. XLIII, p. 307; Frankfurt, ibid., 1894, Bd. XLIII, 

 p. 177. Cf. also Neumeister, Physiol. Chemie, 1893, Bd. I, p. 69. 



a E. Schulze, 1. c., 1895, Bd. XLVI, p. 69. 



8 Literature : E. Schulze, Zeitschr. f. physiol. Chemie, 1890, Bd. xiv, p. 512, and Ubereinstim- 

 mung d. Stoffe im Pflanzen- u. Thierreich, 1894, p. 10; Gerard, Bot. Centralbl., 1892, Bd. L, 

 p. no. 



4 Cf. E. Schulze, Biol. Centralbl., 1896, Bd. xvi, p. 849. [E. Strasburger, Die pflanzlichen 

 Zellhaute, Jahrb. f. wiss. Bot, 1898, Bd. XXXI, p. 511.] 



s Details by Tollens, Handb. d. Kohlenhydrate, 1895, Bd. II, pp. 198, 248 ; E. Schulze, Landw. 

 Jahrb., 1^94, Bd. xxm, p. i ; Zeitschr. f. physiol. Chemie, 1894, Bd. xix, p. 38. Cf. also Sect. 84. 



