THE INDUCTION OF POLARITY 



157 



therefore always develops the attaching organ, but it has not been 

 determined whether this dissimilar polarity is accompanied by a stable 

 physiological verticibasality. The latter is, however, apparently the case in 

 Euglcna and Chlamidomonas^ for here the bipolarity seems to be determined 

 during the process of cell-division. 



The apparently stable verticibasality of the prothallia of Eqnisetum 

 limosum and E. varicgatum is according to Stahl ! induced by light, for 

 the first division-wall is formed at right angles to the incident ray, the 

 exposed cell (p, Fig. 28) being the prothallus cell, and the shaded one (r) 

 forming the first rhizoid. This differentiation induced by light is necessary 

 for the formation of the prothallus, and the normal development is 

 disturbed when the spores are equally illuminated on all sides. 



The protoplasts of many spores are probably either 

 apolar, or possess extremely labile polarity. This might 

 still be the case, even when the germ-tube normally 

 always appears at a certain point, owing to the structure 

 of the external coats. Those pollen-grains in which the 

 exosporium is beset with pores at all points are probably 

 apolar, for the endosporium may protrude at any one 

 of these places. 



The ovum of Fucaceae is fertilized outside the 

 oogonium and does not seem to require an orienting 

 stimulus to determine the direction of the first plane of 

 division which marks off the shoot-segment from that 

 of the root. The ova of Pelvctia canalicnlata and 

 Ascophyllum nodosum have, however, no fixed polarity, for the shoot- 

 segment is always the one exposed to light ~. The ova of Fucus serratus 

 are, on the other hand, inherently bipolar, for Rosenvinge was unable 

 to detect any influence of the external conditions upon the direction of 

 the primary divisions. 



Nor has it been found possible to alter the direction of the planes of 

 divisions with regard to the archegonium in the ova of ferns 3 , although 

 Leitgeb found that a slight change was produced in the case of Marsilea 



V 



FIG. 28. Germinating 

 spore of Eqnisetum. The 

 arrow shows the direc- 

 tion of the incident rays. 

 (After Stahl.) 



1 Stahl, Ber. d. Bot. Ges., 1885, p. 34; Buchtien, Bibl. hot., 1887, Heft 8; Kny, Ber. d. Bot. 

 Ges., 1896, p. 378. 



2 Rosenvinge, Rev. gen. d. Bot, 1889, T. I, p. 126. Farmer and Williams (Phil. Trans., 

 1898, Vol. cxc, p. 641) doubt whether light exercises any such orienting influence. It is also 

 possible that in some species the distribution of oxygen, or the point of entry of the sperm, may 

 determine the polarity of the morphologically radial ovum. Cf. Behrens, Ber. d. Bot. Ges., 1886, p. 92 ; 

 Strasburger, Botan. Practic., 2. Aufl., p. 401. See also Roux, Archiv f. mikr. Anat., 1887, Bd. xxix, 

 P- I . r i7 Winkler, Ber. d. Bot. Ges., 1900, p. 297. In Cystoseira barbata unilateral illumination 

 induces the subsequently fixed polarity. 



3 Leitgeb, Sitzungsb. d. \Vien. Akad., 1879, Bd. LXXX, I, p. 201 ; Heinricher, Mitth. a. d. Bot. 

 lust, zu Graz, 1888, Heft 2, p. 239. 



