224 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



be bent into a different position, the buds upon it to a greater or less 

 distance from the cut surface, now the apex of the shoot, will shoot 

 out. The length and strength of the shoots gradually decrease from 

 the point. If the shoot be bent into a horizontal position then the 

 buds all round the point shoot out, but of the others only those 

 upon the upper side of the twig ; the buds of the under side remain 

 quiescent or only solitary ones develop. The effect of gravity is quite 

 evident here. In the annual shoots of our trees, as is well known, the 

 buds near the apical end, which in the normal condition is the upper, 

 are furthered beyond those at the base, a point which is material to the 

 formation of the whole skeleton of the tree, and in producing this 

 gravity acts as well as polarity. On the thallus of an alga or a liverwort 

 the lateral shoots nearest the apical region are not furthered beyond 

 the basal ones (see, for example, Fig. n) ; the polarity in the regeneration 

 is, as we have seen, conditioned here solely by the direction of the 

 current of plastic material. It is probable that only those plants can 

 develop strong orthotropous shoot-systems in which gravity induces the 

 predominant development of the shoots upwards. 



2. QUALITATIVE INFLUENCE OF GRAVITY. 



What has just been said leads me to speak of the qualitative 

 influence of the stimulus of gravity in the formation of organs. This 

 often acts in concert with correlation. 



The first illustration I shall give is a case investigated by Sachs l . 

 Yucca, Cordyline, and allied plants, possess, in addition to their aerial 

 shoots, thick fleshy rhizomes which grow vertically into the earth. 

 In normal circumstances, that is to say, so long as the aerial stem is not 

 injured, these serve as reservoirs of food-material. If however this stem 

 be cut off, leafy shoots are formed at the upper (basal) portion of the 

 rhizome, but its terminal bud does not shoot out ; but this bud will 

 do so if the rhizome is turned so that the bud is erect. Still, it is 

 not necessary to cut off the aerial parts in order to induce the rhizome 

 to form its shoot. It will develop if the growth of the aerial parts 

 be restricted. This happens usually if the whole plant is placed in an 

 inverted position ; but if the chief shoot then instead of becoming re- 

 stricted in growth erects itself in a negatively geotropic manner, the 

 shooting out of the terminal bud of the rhizome, as well as of the buds 

 at its base, is suppressed 2 . We see then, considering alone the end-bud 



1 Sachs, Stoff und Form der Pflanzenorgane, Gesammelte Abhandlungen, ii. p. 1187. 



2 Vochting, Uber Spitze und Basis an Pflanzenorganen, in Botan. Zeitung, iSSo, confirmed Sachs' 

 observation that the rhizome in normal regeneration behaves in respect of polarity like a root ; his 

 objections to Sachs' explanation are of no account. 



