THE INDUCTION OF DORSIVENTRALITY 151 



upper surfaces of the new growths formed from fern prothalli floating on 

 water and illuminated from beneath. Evidently the older tissues exercise 

 in this case no determining influence upon the new growth l , although 

 a certain directive action may be exercised, which might perhaps suffice 

 to impress the original dorsiventrality upon new growths formed under 

 homogeneous illumination. If any such tendency actually exists it must 

 be very feeble, for a trifling difference in the illumination of the two sides 

 suffices to overcome it. Occasionally rhizoids and sexual organs appear 

 on both surfaces, so that a prothallus might conceivably develop as a bilateral 

 but not dorsiventral structure. The bilateral shoots of Tkuja, for example, 

 develop equally well when no dorsiventral structure has been induced in 

 their tissues, owing to the removal of the unilateral exposure to which they 

 are normally subjected. 



The flattened lateral axes of Thuja occidentalis and other Cupressineae branch 

 in one plane and bear scale-like adpressed leaves (Fig. 25, b). These form 

 four rows, one dorsal, one ventral, and two marginal. A transverse section of 

 such a twig shows usually a general anatomical arrangement corresponding to 

 that of a dorsiventral leaf, for stomata are present on the shaded under surfaces, 

 and palisade parenchyma is present on the upper surfaces exposed to light. This 

 differentiation is directly induced by the unequal illumination of the two surfaces, 

 and merely covering the upper surface with black cloth is sufficient to reverse the 

 dorsiventrality of the new growths, and to weaken that of the parts which were 

 at the time in process of formation 2 . In this last case the leaf-bases are more 

 affected than the leaf-apices, because in young leaves the apices are in a more 

 forward condition of development than the bases. The twigs of Biota orientalis 

 which branch in a vertical plane usually show no dorsiventrality because they are 

 equally exposed on all sides, whereas when subjected to unilateral illumination 

 they also develop the above-mentioned dorsiventral structure 3 . It is therefore 

 evident that a labile, as in Thuja and prothallia, as well as a stable dorsiventrality, 

 as in Marchantia, can be induced by the action of external stimuli, and the mode 

 of response of labile organs to inducing stimuli is the same in character as that 

 of those possessing inherent dorsiventrality. 



The shoots of Thuja,Biota> and Chamaecyparis are especially instructive, 

 for they show that in them and in cuttings taken from them 4 the bilateral 



1 Leitgeb, Flora, 1877, p. 174; 1879, p. 317; Sitznngsb. d. Wien. Akad., 1879, Bd. LXXX, 

 I, p. 201 ; Prantl, Bot. Ztg., 1879, p. 697. Bauke (Bot. Ztg., 1878, p. 771; Flora, 1879, p. 44; 

 Sitzungsb. d. Brandenburg. Bot. Vereins, 1879, p. 121) erroneously supposed that the dorsiventrality 

 was induced solely by gravity. Leitgeb (Ber. d. Bot. Ges., 1885, p. 169) states that the sporophytic 

 buds of apogamous prothalli also appear on the shaded side. 



2 Frank, Jahrb. f. wiss. Bot., 1873-4, Bd. IX, p. 147; Pick, Bot. Centralbl., 1882, Bd. XI, 

 p. 440; Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxii, p. 268. Anatomical details are given by 

 Klemm, Jahrb. f. wiss. Bot., 1886, Bd. xvn, p. 499. 



3 Pick, 1. c. 



* Mohl, Vermischte Schriften, 1845, P- 22> Cf. Frank, 1. c., p. 183, footnote. 



