484 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



as a direct product of the oxidation of carbohydrates. Similarly, the relations 

 between growth and respiration are such as to render impossible the existence 

 of any constant ratio between the amount of cellulose formed and the respiratory 

 activity '. The manner in which calcium influences formation of the cell-wall in 

 certain cases, has already been mentioned (Sect. 74). . 



The various changes which cell-walls may undergo may be either 

 chemical in nature or due simply to impregnation with foreign substances. 

 Both processes may occur simultaneously either during growth or after its 

 cessation, so as to produce local or general changes in the properties of the 

 wall. The latter may from the commencement differ from the normal type. 

 The mucilaginous modification of the cell-wall involves marked chemical 

 metamorphosis, such as occurs during the mobilization of reserve-cellulose, 

 during the formation of gum in Astragalus, &c., and of mucilage in 

 the seed coats of linseed, quince, and sage, and in many gland -hairs. 

 Similar changes occurring in the transverse walls of vascular elements 

 render the formation of vessels possible, and during the copulation of 

 plants of the Conjugatae the dividing walls of the fertilization-tubes undergo 

 corresponding chemical metamorphosis and disintegration. Similarly the 

 partial or complete solution of the middle lamella enables the plant to form 

 intercellular spaces, and to complete the abscission of fruits or leaves 2 . 



Although chemical changes and infiltration co-operate in producing 

 cuticularization and suberization, it is doubtful whether the waxy substances 

 penetrate the cell-wall as such, or are produced in it by chemical meta- 

 morphosis, and the same doubt exists with regard to wood -gum, although 

 lignification is undoubtedly mainly due to chemical changes in the original 

 cellulose (Sect. 83). The calcium carbonate of cystoliths is derived from 

 an organic compound, probably calcium pectatc (Sect. 74), and the character 

 of the cell-wall may be markedly modified by the formation of compounds of 

 this kind. 



Lignification, suberization, and the mucilaginous modification are 

 produced in living tissues only and are due to the vital activity of living 

 protoplasts. Various chromogens, tannins, &c. may penetrate the cell- 

 walls after the death of the protoplast, and may thus take part in the 

 formation of coloured duramen 3 . 



1 For such views, see Sachsse, (Jber chem. Vorgange bei d. Keimung von Pistun sativum, 1873, 

 p. 40, and Chemie d. Farbstoffe, &c., 1877, p. 40. 



a See Sect. 83. Further details in the works of de Bary, Zimmermann, &c. 



s On the formation of duramen, &c., cf. Temme, Landw. Jahrb., 1885, Bd. Xiv, p. 465 ; Prael, 

 Jahrb. f. wiss. Bot., 1888, Bd. XIX, p. 61 ; G. Kraus, Grundlinien z. Physiol. d. Gerbstoffes, 1889; 

 Strasburger, Leitbahnen, 1891, p. 96; Rathey, Ober d. Auftreten v. Gummi i. d. Rebe, 1896; 

 Schellenberg, Jahrb. f. wiss. Bot., 1896, Bd. XXIX, p. 237 ; Hoppe-Seyler, Zeitschr. f. physiol. Chemie, 

 1889, Bd. xin, p. 66; G. Lange, ibid., 1890, Bd. xiv, pp. 15, 217, 283. 



