THE GROWTH OF THE CELL-WALL 29 



A rise of turgidity may be the active cause in producing the com- 

 mencement, or an acceleration, or even a retardation of growth under 

 special conditions. As a matter of fact, however, variations in the rapidity 

 of growth are very often unaccompanied by changes of turgidity. This 

 applies to the acceleration of growth caused by a rise of temperature, or 

 by the absence of light, as well as to the growth-curvatures caused by 

 geotropic and heliotropic stimuli. Similarly no changes of turgor seem to 

 be connected with the retardation of growth caused by the absence of 

 oxygen, or with the cessation of growth during the winter resting period. 

 As the cell increases in volume its turgor is kept constant by a corre- 

 sponding production of osmotic substances 1 , and it depends upon various 

 circumstances whether the percentage of water increases or decreases. 

 During stretching-growth the percentage of water usually increases 2 , but 

 decreases when secondary thickening occurs, while in the case of Asomato- 

 phytes the percentage of water may remain approximately constant. 



Historical. The attempts to discriminate between the different co-operating 

 factors concerned in the growth of organized bodies are mainly based upon 

 observations made upon cell-walls and starch-grains. The first conscious attempts 

 to refer growth to molecular processes were those of Nageli 3 , and his conclusions 

 retain great theoretical importance, although largely based upon the erroneous 

 assumption that the growth of starch-grains takes place by intussusception. Nageli 

 also concluded that the cell-wall usually grows by intussusception, and only rarely 

 by apposition. The predominant importance attached to intussusception was 

 ultimately followed by a reaction to the other extreme, many authors 4 con- 

 sidering that the cell-wall always grew in thickness by apposition, and hence 

 always increased in surface extent by plastic stretching. In the meantime 

 other arguments for intussusception were brought forward by certain authors 5 , 

 and among these Strasburger, who now adopts the conclusion that growth may 

 be brought about in various ways. This view was first put forward by Pfeffer 6 , 



1 Pfeffer, Druck- u. Arbeitsleistungen, 1893, pp. 412, 428; Studien z. Energetik, 1892, p. 245; 

 Noll, Flora, 1895, Erg.-bd., p. 44; Wortmann, Ber. d. Bot. Ges., 1887, p. 461. 



2 Kraus, Ueber d. Wasservertheilung i. d. Pflanze, 1879, I (Festschr. d. naturf. Ges. z. Halle) ; 

 1880, II (Abhandl. d. naturf. Ges. z. Halle, Bd. xv). 



3 Nageli, Die Starkekb'rner, 1858, p. 213. 



4 Schmitz, Sitzungsb. d. niederrhein. Ges. f. Natur- u. Heilkunde, 6. Dec. 1880; Strasburger, 

 Bau. u. Wachsthum d. Zellhaute, 1882 ; Klebs, Unters. a. d. bot. Inst. zu Tubingen, 1886, Bd. II, 

 p. 372 ; Noll, Unters. ii. d. Wachsthum der Zellmembran, 1887, p. 126 ; Wortmann, Bot. Ztg., 1889, 

 p. 230; Zimmermann, Pflanzenzelle, 1887, p. 153 ; Askenasy, Ber. d. Bot. Ges., 1890, p. 85 ; Wiesner, 

 Die Elementarstructur, 1892; Strasburger, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 572. On 

 Cystoliths see Giesenhagen, Flora, 1890, p. 90. 



5 Leitgeb, Bau und Entwickelung d. Sporenhaute, 1884; Wille, Entwickelungsgeschichte d. 

 Pollenkorner d. Angiospermen, 1886; Krabbe, Jahrb. f. wiss. Bot., 1887, Bd. XVIII, p. 346. 

 Cramer, Unters. ii. d. verticillirten Siphoneen, 1890, p. 35 (repr. from Denkschr. d. schweiz. naturf. 

 Ges., Bd. xxxn); Correns, in Zimmermann's Beitr. z. Morphol. u. Physiol., 1893, Bd. I, p. 256; 

 Flora, 1889, p. 289; Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 429; Strasburger, 1898, I.e., &c. 



' Pfeffer, Studien zur Energetik, 1892, p. 240; Druck- u. Arbeitsleistungen, 1893, p. 429. 



