158 THE CAUSES OF SPECIFIC SHAPE 



by the stimulus of gravity l . The first division plane is in this case parallel 

 to the long axis of the archegonium, but is always at right angles to the 

 perpendicular when the archegonium is placed in various horizontal 

 positions. The upper segment in this case always forms the shoot, the 

 lower the root, so that the ovum does not possess any fixed inherent 

 polarity within the limits mentioned. Whether the ova of ferns have a 

 fixed autonomic polarity must remain uncertain, for we do not know the 

 extent to which the archegonium and prothallium influence the direction 

 of the first planes of division. Nor is it known whether the egg-cell in 

 the embryo-sac of a flowering-plant is inherently bipolar, or whether like 

 a meristem cell it is radial in character, and has its polarity determined 

 by interaction with its point of attachment, as in lateral shoots and as in 

 the vegetative embryos of Funkia. In any case it has not been found 

 possible to alter the normal orientation of the embryo by the action of 

 gravity and other external stimuli 2 . 



The above-mentioned polarity is exhibited when roots or shoots are kept 

 under constant external conditions in air saturated with moisture 2 . The polarity 

 of many shoots is hardly at all affected by hanging them upside down, whereas 

 that of others is somewhat weakened. Furthermore, in horizontal pieces of stem, 

 the stimulus of gravity favours the development of shoots from the upper side 

 of the stem-pole and of roots from the under side of the root-pole 4 . 



The region of root-production can naturally be caused to shift towards the 

 stem-pole by diminishing the supply of water or of oxygen at the root-pole, and 

 internal factors may also act against the inherent polarity. Thus owing to the 

 special powers of growth at the nodes, the roots and shoots may at first appear 

 at these points, and further, the strongest buds may develop first even when they 

 are some distance from the shoot-pole 6 . 



Special internal factors are also responsible for the fact that on portions of 



1 Leitgeb, Sitzungsb. d. Wien. Akad., 1878, Bd. LXXVII, I, p. 222. Figures in Goebel's Outlines, 

 1882, p. 259. On the influence of gravity on frogs' eggs cf. O. Hertwig, Zelle u. Gewebe, 189$, II, 

 p. 95. Isolated cells with an excentric centre of gravity naturally assume a position, when in water, in 

 which the centre of gravity lies vertically beneath the centre of gravity of the water they displace. 



2 Voehting, Jahrb. f. wiss. Bot., 1885, Bd. XVI, p. 393; Schmid, Bot. Centralbl., 1894, Bd. 

 LVIII, p. i. 



3 Vochting, Organbildung im Pflanzenreich, 1878, i. Theil ; 1884, 2. Theil ; Bot. Ztg., 1880, 

 P- 593 ; Jahrb. f. wiss. Bot, 1885, Bd. xvi, p. 370 (Marchantia\ and 1899, Bd. xxxiv, p. 36 ; Trans- 

 plantation am Pflanzenkorper, 1892 ; Fr. Darwin, Linn. Soc. Journ., 1880, Vol. xvm, p. 407 

 (fiufius); Wakker, Bot. Centralbl., 1887, Ed. xxxn, p. 238; N. J. C. Miiller, Ber. d. Bot. Ges., 

 1885, p. 159; Kny, Ber. d. Bot. Ges., 1889, p. 201 ; Rechinger, Verh. d. Zool.-Bot. Ges. in Wien, 

 1893, p. 310; Prunet, Rev. ge"n. d. Bot., 1893, T. v, p. 49 (potato). 



4 According to Vochting (1. c., 1878, I, p. 26), Kny (1. c., p. 201), Rechinger (1. c., p. 327% the 

 tendency to formation of callus on the root-pole is due to internal causes, whereas Tittmann (Jahrb. 

 f. wiss. Bot., 1895, Bd. xxvii, p. 193) could detect no such dissimilar polarity as regards callus-forma- 

 tion in Popuhis pyramidalis, but found that a trifling difference of pressure sufficed to determine the 

 end at which callus was formed. 



5 Vochting, 1. c., I, pp. 15, 242 ; II, pp. 59, 130. 



