15 



THE CAUSES OF SPECIFIC SHAPE 



orientation is permanently induced within two or three days, and before any 

 morphological differentiation is perceptible 1 . 



Leitgeb has shown that unilateral illumination induces the permanent 

 dorsiventrality in the germ-filaments growing from the spores of Marchantia, 

 Dircalia^ Grimaldia, and other Marchantiaceae ~. The same probably 

 applies to the strongly dorsiventral foliose Hepaticae, for when the under 

 surface is brightly illuminated the dorsiventrality is slightly weakened but 

 not reversed 3 . It has in all cases to be determined whether the dorsiven- 

 trality can be induced under homogeneous illumination, or whether unilateral 



FIG. 24. A. Gemma of Marchantia polymorpha^ showing the two growing points in the groove at v. B. Later 

 stage, showing the new growths s, .r ; the growing apices p, p\ a, the cells which on the under surfaces develop 

 rhizoids n ; m, insertion of stalk of gemma. 



illumination forms an essential condition for its production. The latter 

 appears to be the case in Marchantia, according to Czapek 4 , for small 

 weakly plants of radial structure developed from gemmae kept slowly 

 rotating on a clinostat for two or three months. It appears from this that, 

 as might be expected, the development of the dorsiventrality is a condition 

 for vigorous growth. 



The equally strongly marked dorsiventrality of fern prothalli is, how- 

 ever, the result of the localized inductive action of the existent external 

 conditions. Thus the rhizoids and sexual organs are developed on the 



1 Pfeffer, Arb. d. Bot. Inst. in Wiirzburg, 1871, Bd. I, p. 77. Mirbel (Mem. d. 1'Acad. d. Sci. 

 de 1'Inst. de France, 1835, T. XIII, p. 337) was unable to determine the internal factors involved. 

 Cf. Pfeffer, Unters. a. d. Bot. Inst. zu Tubingen, 1885, Bd. I, p. 528. That Lunularia behaves 

 similarly to Marchantia has been shown by Leitgeb, Bot. Ztg., 1872, p. 766; Kny, Die Entw. 

 d. Packeriaceen, 1875, p. 12 (Nova Acta d. Leopold. Acad., Bd. xxxvil); Vochting, Jahrb. f. wiss. 

 Bot., 1885, Bd. xvi, p. 378. 



2 Leitgeb, Die Keimung der Lebermoossporen in ihrer Beziehung znm Licht, 1876 (Sitzungsb. 

 d. Wien. Akad., Bd. LXXIV, Abth. l). Cf. also Goebel, Organography, 1900, I, p. 227. 



3 Pfeffer, 1. c., p. 94. 



4 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 261. 



