42 GROWTH AND CELL-DIVISION 



Usually the cell divides into two segments by a wall at right angles to the 

 long axis of the cell. This is, however, not a general law, as Sachs supposed \ 

 and during free cell-formation the dividing lines are not at right angles but 

 follow different lines of equilibrium. In addition, the cell-plate formed across 

 a cylindrical cell which is undergoing simple division may at the outset be 

 obliquely inclined to the long axis of the cell 2 , while the longitudinal divisions 

 of elongated cambium-cells do not follow the law of the smallest possible 

 division-walls (cf. Berthold, 1. c.). Nor is Hofmeister 3 correct in supposing 

 that the last dividing-wall is always at right angles to the direction in which 

 growth was previously most active. This does, however, apply to simple 

 cell-division, so long as the size at which division occurs remains the same, 

 and only the increase in size regulates the divisions. The partition-walls in 

 Spirogyra are formed according to this law, as well as in many cells in which 

 growth and division take place in two or three directions. If in such cases 

 mechanical obstacles render growth possible in one direction only, then the 

 divisions follow the same plan as in a filament of Spirogyra*. 



These relationships remain the same whether cellular and nuclear division 

 take place simultaneously or separately, and the same laws apply to nuclear 

 as to cellular division. Kny (I.e., p. 397) allowed spores of Equisetum to 

 germinate between glass plates, and observed that the axes of the nuclear spindles 

 were parallel and the cell-plates therefore at right angles to the glass plates and 

 to the direction of growth. The same took place when the glass plates were 

 illuminated on one side only, for it was mechanically impossible for the prothallus 

 to respond to light by placing its direction of growth, and consequently the 

 axes of the nuclear spindles, parallel to the incident ray, as it does under 

 ordinary circumstances 5 . It follows from the above that the direction of nuclear 

 division may determine that of cell-division. 



[Kny 6 also finds that pressure may actually induce cell-division in the pith 

 of Impatiens, and that very pronounced pressure will cause the periclinal 

 divisions of the cambium to cease and anticlinal ones to appear. In many 

 cases he finds that pressure induces the formation of cell-walls parallel to its 

 direction, while tension favours the production of walls at right angles to its 

 line of action. Whether the action of tension and pressure upon cell-division 

 is direct or indirect remains, however, still uncertain. ED.] 



these works the physical principles are also discussed. Cf. also Zimmermann, Beitr. z. Morph. u. 

 Physiol., 1893, Bd. I, p. 159. 



1 Sachs, Arbeit, d. Bot. Inst. in Wurzburg, 1879, Bd. II, p. 46; Flora, 1892, p. 63; 1894, 

 p. 221. 



2 See de Wildemann, 1. c., pp. 5, 19, 28, 73 ; and also Berthold, 1. c., p. 244. 



3 Hofmeister, Jahrb. f. wiss. Bot., 1862, Bd. in, p. 272 ; Pflanzenzelle, 1867, p. 129. Hofmeister 

 specially mentioned that cell-division is the result and not the cause of growth, while Sachs put 

 forward the law of division at right angles without seeking any causal explanation of it. 



* Kny, Ber. d. Bot. Ges., 1896, p. 378. Cf. also Pfeffer, Druck- und Arbeitsleistungen, 1893, 

 p. 358. On experiments with animals see O. Hertwig, Zellen und Gewebe, 1893, p. 176; 1898, 

 p. 99; Braun, Biol. Centralbl., 1894, Bd. XIV, p. 340; Zimmermann, Zellkern, 1896, p. 87. 



5 Stahl, Ber. d. Bot. Ges., 1885, p. 334. See also Buchtien, Bibl. Bot., 1887, Heft 8, p. 16. 



6 Jahrb. f. wiss. Bot., 1901, Bd. xxxvn, p. 55. 



