32 THE MECHANICS OF GROWTH 



carbohydrate materials, or by the secretion and external deposition of 

 particles of chitin or cellulose 1 , while any subsequent changes in the cell-wall 

 may also be produced in a variety of ways. These changes may also be very 

 varied in character, without there being any reason for supposing that the 

 cell-wall must be penetrated by living protoplasm 2 . Hence it is possible 

 that in growth by intussusception the interpolated particles of cellulose are 

 derived from the decomposition of proteid and other substances, or are 

 produced by the condensation of carbohydrates, or are deposited by pre- 

 cipitation from a solution permeating the cell-wall. Nothing definite is, 

 however, known as yet, and the production of mucilaginous sheaths around 

 certain Conjugatae 3 , &c., simply shows that materials secreted by the pro- 

 toplast may pass through the cell- wall, and be deposited as an additional 

 external layer which is also composed of a carbohydrate. 



An intimate correlation exists between the different forms of growth, 

 and the thickening 4 , cuticularization, or lignification of the cell-wall usually 

 take place when the external growth of the cell has ceased. These changes 

 serve definite purposes, and are not necessary to produce a cessation of 

 growth, for the latter may also occur in meristematic cells, and no resump- 

 tion of growth can be induced in many cells whose walls have undergone 

 no secondary thickening and no perceptible change in character. A 

 cessation of growth, therefore, does not always result from an incapacity 

 for growth on the part of the cell-wall. Indeed, although no lignified 

 membrane has as yet been observed to grow in surface extent 5 , it is not 

 impossible that exceptions may occur, or that growth might not be 

 resumed in such cells as the result of a retrogressive metamorphosis of 

 the cell-wall. The frequent separation of the cuticle is due to its partial 

 or incomplete inability to grow, but in some cases it appears to have this 

 power 6 . Moreover in certain cases, as for example in the collenchyma at 



1 Strasburger (Jahrb. f. wiss. Bot, 1898, Bd. xxxi, p. 573) has recently brought forward 

 additional instances of varied modes of growth in the cell-wall. Here also the remaining literature 

 is given. Plasmolysis may usually, but not always, be produced in embryonic cells (Reinhardt, 

 Festschrift fur Schwendener, 1899, p. 425 ; Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 307). The 

 latter is also exceptionally the case in adult cells, and hence no certain conclusions can be drawn 

 from this fact as to the growth of the cell-wall, or its production from the peripheral layer of 

 protoplasm. 



8 Cf. Vol. I, p. 484, and the literature there given; also Strasburger, 1. c., p. 558. On extra- 

 cellular protoplasm cf. Schutt, Jahrb. f. wiss. Bot., 1899, Bd. xxxm, p. 594; Bot. Ztg., Ref., 

 1900, p. 245 ; O. Muller, Ber. d. Bot. Ges., 1899, p. 423 ; 1900, p. 492. 



8 Klebs, Unters. a. d. Bot. Inst. z. Tubingen, 1886, Bd. xii, p. 411. 



* Cf. Hofmeister, Pflanzenzelle, 1869, p. 359 ; Klebs, Unters. a. d. Bot. Inst. zu Tubingen, 1888, 

 Bd. II, p. 517. Further details in the works of Wortmann, Zacharias, Sokolowa. On primary 

 meristems cf. Newcombe, Botanical Gazette, 1894, Vol. xix, p. 232. 



6 Schellenberg, Jahrb. f. wiss. Bot, 1896, Bd. xxix, p. 255 ; Warburg, Ber. d. Bot. Ges., 1893, 

 p. 440; Lange, Flora, 1891, pp. 393, 426; Nathansohn, Jahrb. f. wiss. Bot. 1898, Bd. xxxn, 

 p. 683. 



6 Nageli, Starkekorner, 1858, p. 283 ; Schmitz, Bildung und Wachslhum der pflanzl. Zellhaute, 



