INFLUENCE OF EXTERNAL STIMULI. GRAriTY 223 



under side of the horizontal ones. Darkness and moisture have here 

 induced the formation of roots ; but gravity has determined their dis- 

 position on the shoots. Sachs l expressly states this when he says that in 

 all living plants, at least in orthotropous shoots, gravity- acts, in addition 

 to the earlier arising polarity, in such a way that the roots appear on 

 the end which is directed downwards. As a matter of fact we find that 

 formation of roots takes place in progressive serial succession upon the 

 whole under side in most plagiotropic creeping and clinging shoots ; 

 and light also has an effect here in that the roots are arranged upon 

 the under side. 



Let us go back to the formation of organs upon cuttings, and 

 particularly those of willow which have been investigated by Vochting 2 . 

 On cuttings of willow hung up in a vertical position under similar 

 external conditions, buds develop at the apical end, roots round about 

 the basal end. On twigs inclined to the vertical, Vochting found in 

 general the following behaviour a : If the angle which the twig makes 

 with the vertical be small, the growth of buds is chiefly uipon its 

 apical portion and they sprout out from it on every side ; with an 

 increase of the angle of inclination the shoots chiefly form upon the 

 apical portion and upon every side of it, but there is besides an extension 

 of tJie development along the upper side ; finally, if the twig occupies 

 a horizontal position the shoots on all sides of the apex itself sprout 

 out, but behind this only those upon the upper side develop. The 

 growing out of the buds is here caused on the one hand by polarity, 

 which expresses its influence in the preference given to the apical end ; 

 and on the other hand gravity acts, and brings about the formation 

 of the shoots chiefly upon the upper side, whilst the roots appear upon 

 the under. This influence of gravity is well illustrated in Heterocentron 

 diversifolia, a plant whose shoot-axes when used for cuttings possess 

 no primordia of roots but easily form these ; if cuttings of it be hung 

 up in the normal position they produce roots only at the basal end, 

 but if the cuttings be inverted the roots arise at a greater or less 

 distance from the base. 



Similar phenomena to those observed in severed twigs are seen 

 when shoots sprout upon the mother plant, only of course formation 

 of roots is wanting here 4 . If the weak upper part of a nearly erect 

 one-year-old shoot of a willow be severed before the expansion of the 

 buds has taken place, and then the portion of shoot-axis left behind 



1 Sachs, Stoff und Form der Pflanzenorgane, Gesammelte Abhandlungen, I, p. 179, and II, 

 p. 1159; Id. Lectures on Physiology of Plants, p. 520. 



2 Vochting, Uber Organbildung im Pflanzenreich, *. 



3 Vochting, 1. c., p. 169. The exceptions and variations are fully dealt with by the author. 



4 Vochting, 1. c., ii. p. 40. 



