242 TROPIC MOVEMENTS 



which in any case could not apply to the curvatures of unicellular organs. 

 Kohl supposed that the shortening of the cells was due to a rise of turgor 

 in them, of which we have no positive evidence, although certain growth 

 movements are actually produced by active contraction. The activity of 

 the convex side of a curving node of a grass-haulm is well shown by the 

 thickening and bulging it undergoes when curvature is mechanically pre- 

 vented l . Indeed, if the haulm is closely fitted in a glass tube the growth of 

 the under side may be so active in spite of the rectilinear direction enforced 

 upon it as to tear the upper side of the node 2 . 



In the case of the nodes of Triticum, Secale t and many other Grasses, 

 only the sheathing-leaf portion responds geotropically, the central portion 

 being passively bent, whereas in Zea Mays, Saccharum officinarum, both 

 the leaf and stem portions of the node are capable of perceiving and 

 reacting to geotropic stimuli 3 . In Polygonaceae and Commelinaceae, 

 however, the irritability of the leaf-sheath is either slight or absent, so that 

 it is passively bent during curvature. Such cases make clear the fact that 

 the different tissues may not all be equally excitable and responsive, but 

 experiments with isolated tissues leave it uncertain whether the result 

 observed represents the actual part played by the given tissue in the intact 

 organ. Not only may the operation alter or inhibit the irritability of the 

 tissue, but also the removal of the correlating influence of the neighbouring 

 parts may produce a pronounced change of tone. In addition, tissues 

 capable of reaction but not of perception must always appear irresponsive 

 when isolated. 



The removal of the epidermis or of the cells bordering upon it, as well 

 as the removal of the pith and even of the ring of vascular bundles, does 

 not suspend the power of geotropic reaction, whereas the isolated pith is in 

 many cases incapable of any geotropic response 4 . According to Sachs 5 , 

 the pith taken from geotropically curving stems straightens itself, so that 

 the permanent curvature ultimately assumed by the pith may be passively 

 impressed upon it. There is, however, no certain proof in a single case that 

 the pith is incapable of perception, but is able to actively respond to 

 geotropic stimuli transmitted to it 6 . In many cases, however, the cortex 

 of stems and stem-nodes, or portions of it, appears to be especially per- 



1 Cf. Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 396 ; Noll, Arb. d. hot. Inst. in Wiirzburg, 

 1888, Bd. ill, p. 509 ; de Vries, Landw. Jahrb., 1880, Bd. IX, p. 482. 



2 Pfeffer, Ber. d. Sachs. Ges. d. Wiss., 1891, p. 642. 



3 Earth, Die geotropischen Wachsthumskriimmungen der Knoten, 1894; Pfeffer, 1. c., 

 pp. 390, 409. 



1 Sachs, Flora, 1873, p. 330 ; Earth, 1. c., p. 36 ; Czapek, Jahrb. f. wiss. Eot., 1898, Bd. xxxii, 

 p. 248 ; Haberlandt, Ber. d. bot. Ges., 1901, p. 269; Nemec, ibid., 1902, p. 339. 



5 Sachs, Experimentalphysiol., 1865, P- 5<>7. Cf. also Frank, Beitrage zur Pfknzenphysiol., 

 1868, p. 73. 



8 Cf. Haberlandt, 1. c., p. 269. 



