THE COMPOSITION OF THE CELL-WALL 481 



on the one hand and gums or mucilage on the other, so that none of these 

 groups can be strictly marked off from the others, especially since bodies 

 belonging to the different groups often form compounds with one another. 

 Similarly no sharp line of demarcation can be drawn between the hemi- 

 celluloses and the insoluble pectin substances which are present in every 

 cell-membrane and which may in some cases even predominate. Pectins are 

 probably derived from carbohydrates by the addition of other substances 

 to the molecule of the latter. The changes of lignification and suberization 

 are probably produced by molecular rearrangements and by union with 

 other substances. 



The cell- walls of fungi may either be composed of cellulose, or may 

 contain chitin ; in some cases they are almost entirely composed of this 

 gluco-proteid, which may be regarded as a carbohydrate derivative 1 . In 

 various animals chitin may be associated with a little cellulose, while pure 

 cellulose occurs in the Tunicata 2 , so that it is evident that no essential 

 general difference exists between plants and animals in this respect, 

 although while carbohydrates are mainly employed in plants for the 

 formation of the cell-wall, chitin and various proteids subserve for the most 

 part the same function in animals. 



It is highly improbable that in both plants and animals the cell- 

 membrane is always originally composed of the same substances ; as 

 a matter of fact fungi seem to form their chitinous membranes directly, 

 and not by secondary modification, while the layers which serve as 

 reserve-cellulose are deposited from the first in the form of galactans, 

 mannans, &c. 3 Indeed it will probably be found by closer research that 

 cell-walls or layers of cell-walls are often formed which even in the 

 moment of their formation do not consist of cellulose. It is, however, 

 always possible that the cell- membranes when once deposited may later 

 undergo a variety of subsequent metamorphoses. 



As our chemical knowledge increases, the distribution of the component 

 elements of cell-membranes may be detected by microchemical means, not only 

 throughout the plant as a whole, but also in each individual cell- wall. The 

 present position of affairs may be seen by reference to Zimmermann's Mikro- 

 technik, p. 135. By the conjoint aid of microscopical and microchemical methods 



1 Winterstein, Zeitschr. f. physiol. Chem., 1894, Bd. xix, p. 521 ; 1895, Bd. xxi, p. 134; Ber. 

 d. Bot. Ges., 1895, p. 65; Gilson, La Cellule, 1894, T. xi, p. 5; Bot. Centralbl., 1895, Bd. LXI, 

 p. 289; Tollens, I.e., p. 255, where the remaining literature is given. On fungal cellulose, cf. 

 de Bary, Morph. u. Biol. d. Pilze, 1884, p. 9 ; Zimmermann, Mikrotechnik, 1893, p. 157. On the 

 membranes of lichens, see Escombe, Zeitschr. f. physiol. Chemie, 1896, Bd. xxil, p. 288. The 

 exosporium and endosporium of many spores probably contain nitrogen. [C. von Wisselingh, 

 Mikrochem. Unters. iiber die Zellwande d. Fungi, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 619.] 



2 Ambronn, Mitth. a. d. Zool. Station in Neapel, 1890, Bd. ix, p. 475. On chitin, &c., cf. Neu- 

 rheister, Physiol. Chemie, 1893, Bd. I, p. 38. 



3 Cf. Griiss, Bibliotheca botanica, 1896, Heft 39, p. 13. 



PFEFFRR I 1 



