154 THE CAUSES OF SPECIFIC SHAPE 



the aid of some external stimulus, or whether it is an hereditary property 

 developed under all circumstances, and the external stimulus simply exercises 

 a directive orienting action. 



As is generally the case, the same end may be attained in various ways, and 

 a special discussion of the causes influencing symmetrical shape is given by Goebel 1 . 

 In the shoots of Goldfussia anisophylla and Centradenia floribunda the anisophylly 

 is inherent, whereas in Goldfussia isophylla, Centradenia rosea, and Selaginella 

 sanguinolenta it is locally induced and therefore reversible 2 . In Selaginella 

 Krausiana the dorsiventrality and anisophylly can only be somewhat weakened 

 by illuminating the ventral surface of the growing apices. The dorsiventrality 

 of many species of Lycopodium is however induced by light, while light and 

 gravity combined induce the localized dorsiventrality of Hedera helix*. It is 

 however often the case in leaves that pronounced anisophylly, dorsiventrality, and 

 asymmetry are due to internal causes. The same applies to certain inflorescences, 

 whereas in others the dorsiventrality is induced by such stimuli as light and gravity 4 . 

 The rhizome of Nuphar luteum is radial when buried in mud, but becomes dorsi- 

 ventral on exposure to light. The stems of various mosses appear to show all stages 

 of transition from locally to permanently induced dorsiventrality. Various external 

 stimuli, mainly that of light, appear to play a more or less important part in 

 inducing the differentiation of the sporogonium 5 . 



Stahl 6 has observed that unilateral illumination influences the shape of the 

 lichen Endocarpon to a remarkable extent, and it would be of interest to know 

 the factors involved in the case of such a symbiotic organism. 



SECTION 44. The Induction of Polarity. 



Such apolar spherical organisms as Pleurococcus and Micrococcus have 

 no main axis of symmetry, and even in such cylindrical plants as Bacillus or 

 Spirillum the opposite poles are similar in character. All the higher plants 



1 Goebel, Organography, 1900, I, p. 84 seq. ; Wiesner, Sitzungsb. d. Wiener Akad., 1868, 

 Bd. LVIII, I, p. 382 ; ibid., 1892, Bd. ci, I, p. 694; Ber. d. Bot. Ges., 1895, p. 491 ; ibid., 1896, 

 p. 180; Frank, Bot. Ztg., 1868, p. 873; Jahrb. f. wiss. Bot, 1873-4, Bd. ix, p. 185; Die natiirl. 

 wagerechte Richtung von Pflanzentheilen, 1878, p. 34; Pfeffer, Arb. d. Bot. Inst. in Wiirzburg, 

 1871, Bd. I, p. 77; Goebel, Bot. Ztg., 1880, p. 839; Rosenvinge, Rev. ge"n. d. Bot., 1889, 

 I, P- 535 Weisse, Ber. d. Bot. Ges., 1895, p. 376 ; 1896, p. 96 ; Figdor, ibid., 1897, Generates., 

 p. 70; Czapek, Flora, 1898, p. 427. 



2 Wiesner, Frank, and Weisse state that in horizontally placed shoots the leaves on the upper 

 surface are often somewhat smaller than usual. 



8 Sachs, Arb. d. Bot. Inst. in Wiirzburg, 1879, Bd. II, p. 257; Czapek, Sitzungsb. d. Wien. 

 Akad., 1895, Bd. CIV, I, p. 41. 



4 Goebel, 1. c., p. 128 ; Ricome, Ann. d. sci. nat, 1899, 8 e se>., T. vn, p. 293. 

 6 Raciborski, Flora, 1894, p. 32 ; Goebel, 1. c., p. 242. 



6 Wichura, Jahrb. f. wiss. Bot., 1860, Bd. II, p. 197 ; Goebel, Flora, 1895, p. 459; 1896, p. 480; 

 Organography, 1900, I, p. 239; Correns, Bot. Festschrift fur Schwendener, 1899, p. 395. 



7 Stahl, Beitr. z. Entwickelung d. Flechten, 1877, II, p. 18. Observations on the influence of 

 light and other agencies upon the symmetry of algae have been made by Berthold, Jahrb. f. 

 wiss. Bot., 1882, Bd. xm, p. 569 ; Noll, Arb. des WUrzburger Inst., 1888, Bd. II, p. 472 ; Reinke, 

 Ueber Caulerpa, 1899, p. 57; Goebel, Organography, 1900, I, p. 238. 



