THE INDUCTION OF DORSIVENTRALITY 153 



more or less hypertrophy 1 . In all these cases we are dealing with 

 localized induction, for if a shoot is twisted so that the dorsal side faces 

 downwards the dorsiventrality is reversed 2 . According to Frank the same 

 takes place in darkness, but nevertheless under normal conditions the action 

 of gravity apparently co-operates with that of light in inducing the dorsi- 

 ventrality 3 . 



The dorsiventrality of the individual needle-leaves of Taxns is due to 

 internal causes, as in most foliage leaves, but in Thuja we have an instance 

 of local aitionomic and labile induction. In the absence of any continued 

 apical growth it is usually impossible to say whether the preformed parts 

 would impress their own arrangement upon the new growths, although this 

 is actually the case in the growing leaf of Lygodium. Little or no such 

 action comes into play during the limited period of growth of flowers, whose 

 symmetry is mainly determined by internal causes. Nevertheless Vochting 4 

 has shown that the dorsiventrality of the flowers of Epilobium ang-iistifolinm, 

 Hemerocallis fulva, and Clarkia pulchella, whose primordia are radially 

 arranged, is induced by the action of gravity, whereas in Amaryllis 

 formosissima the autonomic dorsiventrality is increased or diminished 

 according to the position of the flower with regard to the earth. 



When a primordium grows apically, the apical meristem is subjected 

 to the directive influence of the specialized tissues developed from it. In 

 case this action is exercised only over a short distance it can be eliminated 

 by the interpolation of a small portion of non-dorsiventral tissue. This 

 happens during the development of the gemmae of Marchantia, and 

 apparently the stalk of the sporangium of Selaginella serves this purpose. 

 If the primordium of a sporangium produces a leafy shoot on a cutting, it 

 probably has the subsequently inherent dorsiventrality induced in it by 

 some external stimulus, such as unequal illumination during the process of 

 conversion 5 . Once the inductive action becomes fixed it acts as though 

 due to internal causes, and hence all side-shoots of Selaginella and of 

 Marchantia appear as though inherently dorsiventral. Even when the 

 dorsiventrality or polarity is a normal property of the organism, the primary 

 induction may be due to an external stimulus. It then needs in every 

 case to be determined whether the dorsiventrality is not acquired without 



p. 22; Kny, Bot. Ztg., 1873, p. 434; Czapek, Jahrb. f. wiss. Bot., 1893, Bd. XXXII, p. 267. On 

 anisophylly cf. also Meissner, Bot. Ztg., 1897, p. 203; 1901, p. 25. 



1 Cf. Czapek, 1. c., p. 268. 



2 As the result of inductive action Kny (1. c.) finds that the anisophylly is not at first reversed 

 when buds expand in an inverted position. 



3 Goebel, Organography, 1900, I, p. 251. 



4 Vochting, Jahrb. f. wiss. Bot., 1886, Bd. xvn, p. 297; Hildebrand, Ber. d. Bot. Ges., 

 1886, p. 329 (Ckome]; Goebel, 1. c., p. in. Differences of illumination may be effective in certain 

 cases. 



5 Behrens, Flora, 1897, Erg.-bd., p. 163. 



