260 TROPIC MOVEMENTS 



evident that de Vries * was incorrect in supposing that the twisting of the internodes 

 was due to the mechanical action exercised by the weight of the leaves. The 

 absence of torsion in the internode when the pair of leaves are removed may be due 

 to a change of tone, or to the cessation of the directive influences radiating from the 

 leaf. According to de Vries, the torsion of the internode of Philadelphus is inhibited 

 by the removal of the upper but not by that of the lower leaf. This requires further 

 investigation, however, as does also the absence of torsion in the defoliated branches 

 of Ulmus and Celtis 2 , since Czapek 3 found that similarly treated branches of Taxus 

 and Picea do undergo torsion 4 . 



INTENSE LIGHT or direct sunlight causes many photometric leaves to 

 rise, sink, or twist in such fashion as to place their laminas or those of 

 the leaflets more or less parallel to the incident rays. This is especially 

 well shown by the compound leaves of Mimosa pudica in which the primary 

 pulvini set the plane of the leaf during the daytime in a plagiotropic position 



FlG. 49. A horizontal shoot of Diervilla lonicera. From the edges of the stem it can be seen that the 

 torsion is completed in the internodes i, 2, and 3, while internode 4 is still straight. 



which is the resultant of the diageotropic and diaheliotropic irritability. The 

 pulvini of the leaflets are, however, able to perform one movement only, and 

 this is photonastic in character. In ordinary light the leaflets are expanded, 

 in darkness and in intense light they close. The latter movement depends 



1 See Noll, Arb. d. bot. Inst. in Wurzburg, 1885-7, Bd - in, p. 358; Schwendener u. Krabbe, 

 1. c., p. 320. 



2 De Vries, 1. c., p. 272. Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxii, p. 288. 



* On the orientation of Mosses and their protonomata see Coesfeld, Bot. Ztg., 1892, p. 192 ; 

 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 265 ; Correns, Festschrift f. Schwendener, 1899, 

 p. 385- A summary of the orienting movements of flowers is given by Noll, Arb. d, bot. Inst. in 

 Wurzburg, 1885-7, Bd. ill, pp. 189, 315. See also Wiesner, Biol. Centralbl., 1901, Bd. XXI, p. 801 ; 

 the quoted works of Schwendener u. Krabbe, Oltmanns, Czapek, as well as Vochting, Jahrb. f. wiss. 

 Bot., 1886, Bd. xvn, p. 297; 1893, Bd. xxv, p. 179; Schaffner, Bot. Centralbl., 1898, Bd. LXXVI, 

 p. 22 (Helianthus) ; Meissner, Bot. Centralbl., 1894, Bd. LX, p. i. 



