THE INDUCTION OF POLARITY 159 



leaves, and of stems with limited growth, not only roots but also shoots may arise 

 at the basal end *. The purpose of this is obvious in the first case, for the leaf- 

 tissue could hardly serve as a channel of communication between the stem and root. 



The same tendency to root or shoot formation is shown not only when an organ 

 is completely separated, but also when the continuity of the bark and phloem is 

 entirely destroyed 2 . 



Reversal experiments. Neither stem nor root can carry on its functions so 

 well when inverted as when in the normal position, and this although water and 

 food-materials can pass in both directions in an intact plant. When an inverted 

 stem has been caused to produce roots at the stem-pole and shoots at the root-pole, 

 it never grows well and ultimately dies 3 . From the literature given by Vochting 

 it can be seen that it is extremely doubtful whether a tree planted in the inverted 

 position has ever lived for any length of time. Owing to the inherent polarity, 

 inverted pieces of stem strive to produce new shoots from the shoot-pole buried 

 in earth. By removing these shoots Kny succeeded in growing inverted plants 

 of Hedera helix and Ampelopsis hederacea for four years, at the end of which time 

 it was found that no reversal of polarity had taken place in the new wood and 

 bark formed in the inverted position 4 . 



The polarity of cells. The above reactions are due to the influence exercised 

 by the adult tissues upon the growing meristem cells, which are themselves equi- 

 potential. This is at once shown by the fact that in cutting pieces from stem or root 

 any pair of points may be made the stem-pole and root-pole respectively, and thus 

 any group of meristem cells may be caused to develop into either a root or a shoot 

 according to which end of the segment contains them. Although the meristem 

 cells therefore possess no fixed polarity, it has still to be determined whether they 

 are entirely apolar and radial, or whether they possess a labile polarity readily 

 influenced by external stimuli. In any case, however, the interaction with the 

 differentiated cells induces in them local differences and a corresponding definite 

 polarity. It is even possible that this induced polarity may be retained by the 

 cells with greater or less energy, so as to be apparently inherent to them. For 

 example, each new cell of the asomatophyte Spirogyra has its main axis of growth 

 and elongation determined by internal causes. 



Nevertheless the polarity is only a labile one, for if the filament is unable to 

 elongate owing to its being imbedded in a plaster-cast, the side walls may bulge 

 out wherever any space is available and form a branch at right angles to the 

 original main axis 5 . 



A labile polarity will remain fixed so long as the conditions inducing it persist, 

 without the embryonal cells necessarily acquiring any fixed inherent polarity. 



1 Vochting, 1. c., 1878, p. 92; Bot. Ztg., 1880, p. 603 ; Transplantation, p. 30 ; Jahrb. f. wiss. 

 Bot., 1885, Bd. xvi, p. 387 (the inflorescence stalk of Marchanticf). 



3 Id., 1. c., 1878, 1, p. 40, &c.; 1884, p. 119; Czapek, Sitzungsb. d. Wien. Akad., 1897, Bd. cvi,I, 

 p. 161. 



3 Vochting, Organbildung, 1878, I, p. 198. 



4 Kny, Ber. d. Bot. Ges., 1889, p. 201. 



5 According to the author's hitherto unpublished observations. Cf. also Pfeffer, Druck- u. 

 Arbeitsleistungen, 1893, pp. 240, 385. 



