156 



THE CAUSES OF SPECIFIC SHAPE 



as root and shoot poles respectively, and we may compare the phenomenon 

 with the behaviour of a magnet when broken into pieces, for each retains 

 a N. and S. pole and these lie at the same ends of each piece. Both root 

 and shoot are so constructed that when inverted they cannot carry on the 

 functions of translocation and correlation as well as normally. This is at 

 once shown by experiments in which the shoot-pole is caused to develop 

 roots, and the root-pole, a leafy axis. 



Probably in many cases, especially in plants with little or no tissue 

 differentiation, it will be found that an adult portion of the shoot can serve 

 equally well, when inverted, as a channel of communication between the 



root and upper portion of the 

 plant. Noll * in fact finds this 

 to be actually the case in the 

 non-septate plant Bryopsis 

 muscosa (Fig. 27), for if in- 

 verted, and the stem-apex 

 with the leaf-members near to 

 it buried in sand, they develop 

 into a root-system, whereas 

 from the older leaf-members, 

 from the base of the stem, or 

 even from the roots, the orig- 

 inal branching stem-axis is 

 restored. Furthermore such 

 algae as Spirogyra and other 

 floatingTorms have no inherent 

 verticibasality, for they only 

 form attaching rhizoids and 

 exhibit dissimilar poles under 

 special external conditions. 

 Even in flowering plants the 



conversion of a root-apex into that of a shopt would probably be a 

 common occurrence were it not for the energetic determining influence 

 exercised by the differentiated tissues. 



Flowering plants possess therefore a relatively stable verticibasality, 

 whereas this is extremely labile in Bryopsis, and it is hence questionable 

 whether it is stable in the bipolar zoospores of this alga 2 . The asexual 

 zoospores of Oedogonium attach themselves by the pigmented end which 



1 Noll, Arb. d. Bot. Inst. in Wiirzburg, 1888, Bd. in, p. 468. Similar results were obtained 

 with Caulerpa prolifera by Noll, 1. c., p. 470; Janse, Jahrb. f. wiss. Bot., 1890, Bd. xxi, p. 237 ; 

 Klemm, Flora, 1893, p. 460; Winkler, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 449; Noll, Ber. d. 

 Bot. Ges., 1900, p. 445. The inductive action is mainly due to light. 



3 Pringsheim (1871), Gesammelte Abhandl., 1895, I, p. 115. 



FlG. 27. Bryopsis muscosa. A Normally erect plant. Z?.'Apex 

 of an inverted plant forming rhizoids. w The shaded part was 

 present before inversion, the unshaded grew after it. k = sand- 

 grains ; b leaf-members ; s= apex of stem. (After Noll.) 



