482 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



it may be possible in part to determine whether the chemical products of the 

 decomposition of the cell-wall are derived from uniformly similar molecules or 

 not. It must always be remembered that solvent agents may not only remove 

 impregnating substances but may also induce decomposition, which probably 

 occurs whenever the cellulose reaction is given only after the application of 

 drastic modes of treatment. 



Hemicelluloses frequently function as reserve-materials in seeds, and yield 

 as the main products of metabolic disintegration mannose and galactose, and in 

 part pentoses, such as arabinose and perhaps xylose also. Hemicelluloses are 

 presumably complex carbohydrates, but according to microscopical observations 

 made during the dissolution of the deposited layers, different varieties may be 

 present '. Although reserve-cellulose is usually composed of hemicelluloses which 

 are easily dissolved, plants are also able to reabsorb typical cellulose (viz. during 

 conjugation, the penetration of fungi, &c.). In algae, however, even substances 

 which are readily soluble, and which may be extracted by means of hot water 

 from the cell-walls, may remain intact during starvation. Since the macrochemical 

 properties of the pectins, which are allied to hemicelluloses, are but little known, 

 it is not possible to localize them microchemically. Hence it is uncertain whether 

 all Mangin's 2 pectin substances do actually belong to this group, although pectic 

 substances seem to be very widely distributed in cell-walls, or in special layers 

 of the cell-wall. 



Lignificatioit*. This probably involves molecular changes and combinations 

 together with infiltration by gum, mineral constituents, &c., for the reactions 

 of lignified walls are such as to indicate that the cellulose, pentose, and aromatic 

 substances are present in the form of more resistant compounds. It is doubtful 

 whether ether-like compounds with lignic acid are formed, or whether there are 

 other molecular combinations. The lignin reactions are partly due to the aromatic 

 groups, and partly to the pentosans present. There is also positive evidence to 

 show that cuticularization and suberization are not merely due to impregnation 

 with waxy substances, but involve as well a formation of special compounds of 

 cellulose from the original constituents of the cell-wall. 



SECTION 84. The Formation and Modification of the Cell-wall. 



The metabolic origin of the different constituents of the cell-wall is not 

 as yet precisely known, nor is it certain in what way they are deposited, 

 whether they occur peripherally as in the case of swarm-spores and plasmo- 

 lyzed protoplasts, or in the interior of the plasma as in typical cell-division 4 . 



1 See E. Schulze, Zeitschr. f. physiol. Chemie, 1895, Bd. xxi, p. 392, and Ber. d. Bot. Ges., 1896, 

 p. 66; also Griiss, Bibl. hot., 1896, Heft 39; Bot. Centralbl., 1897, Bd. LXX, p. 242; Elfert, Bibl. 

 hot, 1894, Heft 30; Nadelmann, Jahrb. f. wiss. Bot., 1890, Bd. XXI, p. 609; Keiss, Landw. Jahrb., 

 1889, Bd. xvni. 



* Mangin, Rech. anat. s. 1. composes pectiques, 1893. Cf. Zimmermann, Mikrotechnik, p. 162. 



5 For the literature, see Tollens, I.e., 1895, p. 270; 1888, p. 239; Zimmermann, Mikrotechnik, 

 p. 140. 



4 Cellulose membranes may be formed around crystals (Wittlin, Bot. Centralbl., 1896, 



