160 THE CAUSES OF SPECIFIC SHAPE 



Hence although grafts only unite when the root-pole of the graft is joined to 

 the stem-pole of the stock, this does not prove, as Vochting supposes, that the 

 growing cells of the two parts themselves possess any fixed inherent polarity 

 preventing fusion unless the proper ends are joined '. 



Although we may agree in ascribing a labile polarity to the meristem cells, 

 we have still to learn by what properties and changes in the protoplast this polarity 

 is produced and modified. It is possible that the polarity of the protoplast itself 

 is unchangeable, and that an external stimulus simply causes it to change its axis 

 of polarity with regard to the fixed cell-wall. The same might, however, be equally 

 well attained by an alteration in the internal structure without any movement of 

 the entire protoplast being required. The latter is apparently the case in the 

 swarm-cells of Myxomycetes in which the cilia may arise at different points, thus 

 showing a variable polarity. Probably also the induced polarity of Spirogyra cells 

 and of the meristem cells of Phanerogams primarily involves a change in the internal 

 structure of the protoplast. 



The origin of polarity. Sachs 2 supposed that physiological verticibasality 

 was directly due to the vertical position of the plant, being induced by the action 

 of gravity or light. The drooping branches of weeping willows 3 and branches kept 

 in inverted positions retain their original verticibasality in spite of the action 

 of gravity 4 . Nor is it surprising that the positively geotropic rhizomes of Yucca 

 and Cordyline should have the same verticibasality as roots 5 , and therefore 

 when a rhizome of this kind produces a negatively geotropic shoot, it undergoes 

 the same alteration as when a root-apex is converted into a shoot. Geotropism, 

 heliotropism, and polarity are independent properties and may alter independently of 

 one another. Hence it is impossible to say what part has been played by gravity 

 and by light in the phylogeny of polarity 6 . 



1 Vochting, Transplantation, 1892, p. 149. The corresponding poles of verticibasal swarm- 

 cells fuse during copulation. Vochting only worked with meristem cells, which owing to inductive 

 action form xylem only internally, although the cambium can actually produce phloem internally. 

 Cf. Vochting, 1. c., p. 146; Beyerinck, Wurzelknospen u. Nebenwurzeln, 1886, p. n ; Frank, 

 Krankheiten d.Pflanzen, 1894, 2. Aufl., Bd. I, p. 70. Miehe (Flora, 1900, p. 113) finds that centrifugal 

 and wound stimuli may cause the mother-cell of the sloma of Monocotyledons to be cut off from 

 the basal instead of from the apical end of the initial cell. 



2 Sachs, Arb. d. Bot. Inst. in Wiirzburg, 1880, Bd. n, pp. 469, 484. Cf. Vochting, Bot. Ztg., 

 1880, p. 593; Organbildung, 1884, Heft 2, pp. 95, 188. 



3 Vochting, Bot. Ztg., 1880, pp. 598, 605 ; Organbildung, 1884, II, pp. 95, 188. The same 

 applies to horizontal rhizomes, potato-tubers, and the like. Cf. Vochting, 1. c., and Bibl. bot., 1887, 

 Heft 4, p. 40. 



* Vochting, 1884, 1. c., p. 132 : Transplantation, 1892, p. 34. 



5 Sachs, 1. c., 1880, p. 475 ; Vorlesungen iiber Pflanzenphysiol., 2. Aufl., 1887, p. 536; Vochting, 

 Bot. Ztg., 1880, p. 601 ; Organbildung, 1884, II, p. iSS. 



6 Cf. also Vochting, Transplantation, 1892, p. 158. 



