240 TROPIC MOVEMENTS 



paper measures, and observed in many cases only a slight retardation of 

 the average rate of growth, while more especially in thick and slowly 

 growing stems and peduncles a more or less pronounced shortening took 

 place on the concave side. Similar results were obtained by Earth with 

 nodes which show growth previously to geotropic excitation l , whereas the 

 nodes of grasses always shorten considerably on the concave side, which 

 undergoes compression. Sachs 2 observed that the thick nodes of cinquantino 

 Maize shortened from 4-3 to 2-5 mm. on the concave side during geotropic 

 curvature, and lengthened from 4-1 to 9-0 mm. on the convex side. The 

 more slender nodes of other grasses shorten but little on the concave side 

 when the curvature is moderately pronounced, so that the neutral axis, 

 which neither elongates nor contracts, lies near to the concave surface. 



No cell-division accompanies the awakened growth of the nodes of 

 grasses, the individual cells increasing in size by stretching growth 3 . The 

 same is shown during the geotropic curvature of roots, so that, as Frank 

 first observed, the cells are longer on the convex than on the concave side 4 , 

 and this holds good even when the curvature is accompanied by cell-divi- 

 sion. When the concave side is compressed, as in pulvini and grass-nodes, 

 the diameter of the cells will in general tend to increase, but not, or only to 

 a slight degree, when growth is retarded without any compression. Kohl 5 , 

 however, observed that during the geotropic curvature of stems the cells of 

 the concave side, and Noll that those of the convex side, attained a rela- 

 tively greater diameter, so that individual peculiarities may occur. The 

 varying growth of strips of equal length marked on straight and curving 

 stems corresponds to what might be expected, that from the concave side 

 being shorter and from the convex side longer than that from a stem in 

 which growth was rectilinear G . According to Miiller 7 , the altered rates of 

 growth in positively heliotropic stems and in negatively heliotropic aerial 

 roots during curvature correspond to those observed during geotropic 

 curvature, so that the same considerations may possibly apply to all forms 

 of tropic curvature produced by growth. 



Each lamella assumes during growth a rate of growth proportionate 



1 Earth, Die geotropischen Wachsthumskriimmungen d. Knoten, 1894, p. n. 



2 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1872, Bd. I, p. 206; Pfeffer, Druck- u. Arbeits- 

 leistnngen, 1893, p. 393; Earth, 1. c., p. 31. Pfeffer and Earth used microscopes with micrometer 

 eyepieces. 



3 Sachs, 1. c., p. 207. 



* Frank, Beitrage z. Pflanzenphysiologie, 1868, p. 40; Cisielski, Cohn's Beitrage z. Biologic, 

 1872, Bd. I, Heft 4, p. 18 ; Sachs, Arb. d. bot. Inst. in Wiirzburg, 1873, Bd. I, p. 466; Macdougal, 

 Botanical Gazette, 1897, Vol. XXIII, p. 364. 



5 Kohl, Mechanik der Reizkriimmung, 1894, P- 5- Cf. also Sachs, 1. c., pp. 462, 469 ; 

 Cisielski, 1. c., p. 18. 



6 Noll, Arb. d. bot. Inst. in Wiirzburg, 1888, Bd. ill, p. 526. 



7 Sachs, Arb. d. bot. Inst. in Wiirzbnrg, 1872, Bd. I, p. 193; Experimentalphysiologie, 1865, 

 p. 507. Cf. also Frank, 1. c., p. 67. 



