286 



NATURE 



[December 12, 1918 



air, can be repeated with this plant. It is found 

 that the apical buds begin to grow first, while none 

 of the others do SO. The result, as pointed out, 8 

 may be explained in two ways : cither a bud, when 

 it begins to grow, sends out inhibitory substances 

 towards the base of the stem, or the young grow- 

 ing bud absorbs the whole of some material re- 

 quisite for growth. This latter explanation appears 

 to bold in the case of the leaf buds, as will be seen 

 later; but the following experiment shows that the 

 former is the correct one for the stem : A piece of 

 Stem is laid horizontally. All the leaves are re- 

 moved, with the exception of the two at the apical 

 node, the petioles of which are left. So lout; as 

 these petioles remain, the buds i'n their axils do not 

 grow, but those in the next node below will com- 

 mence to grow, and when the petioles have wilted 

 and fallen off, the apical buds also begin to grow. 

 Now, if the inhibiting effect of the apical buds on 

 the more basal ones is due to the circumstance 

 that those which grow out first take all the avail- 

 able supply of some necessary material, the buds 

 in the node behind the apical one, since they grew 

 out first, should continue to outstrip in growth 

 those of the apical node which started later. If, 

 on the other hand, the effect is due to an inhibitory- 

 substance sent towards the base by the growing 

 bud, the most apical bud should soon outstrip those 

 behind, because the apical buds send out inhibitory 

 substances towards the next lower buds, while they 

 themselves receive none. The actual result is in 

 favour of the latter view. As soon as the petioles 

 have dropped off, their axillary buds begin to grow, 

 and not only rapidly outstrip in size those below, 

 but even retard or stop their growth altogether. 



If the effect is due to an actual chemical sub- 

 stance, the inhibitory effect should be in propor- 

 tion to the size of the growing bud. It is scarcely 

 possible to test this with the apical bud itself, but 

 it i.m be done with the grown leaf, which als© 

 sends out similar material. This is carried by the 

 sap which flows in the same half of the stem as 

 that to which the leaf belongs. A stem is again 

 laid horizontally, and all the leaves are removed 

 except the two of the apical node. No shoots are 

 produced so long as the leaves remain. If one leaf 

 is removed, its axillary bud grows. The following 

 experiment is then made : Twelve stems are taken, 

 and all the leaves except one of the apical ones on 

 each stem are removed, together with the free 

 axillary bud opposite the leaf which remains. Six 

 stem plai ed with the leaf upwards, six with 



the leaf downwards. In the former case no shoots 

 develop, except sometimes that in the second node 

 behind the leaf, on the lower .side only. In the 

 latter case - 1 I develop either in the two lateral 

 buds of the node behind the leaf or on the upper 

 side of the second node behind the leaf. The inter- 

 pretation is that the inhibiting substances, while 

 flowing on the side he tem where the leaf is, 

 have also a tendency to flow downwards by 

 gravity. If the stem behind the leal contains only- 

 one node, no shoot grows, even when the leal is 



^ " The Chemical Basis of Axial Polarity in Regeneration." By Jacques 



I.oeb. Science, N.S.i vol. Jclvi., p. 547. (1017.) 



NO. 2563, VOL. I02] 



below the node. A large leaf sends out a large 

 enough mass of inhibitory substance to flood the 

 buds. But if the leaf is reduced to one-tenth its 

 size, then shoots are formed in the node behind 

 the leaf. That the inhibitory effect of the leaf is 

 not due to its withdrawing nutritive material from 

 the stem is shown by the fact that the petiole left 

 attached to a leaf remains fresh, but wilts if left 

 attached to the stem without the leaf. The nutri- 

 tive material is therefore sent by the leaf to the 

 stem, not vice versa. 



The fact that it is the apical bud that begins to 

 grow first is given the following tentative explana- 

 tion. When the leaves are first removed, the in- 

 hibitory substances are present everywhere, but 

 continue to tlow towards the base. Hence the most 

 apical node is the first one to become freed from 

 their presence, and when its buds grow they form 

 anew the inhibiting substance which prevents the 

 growth of the more basal buds. 



No reference has yet been made to the collateral 

 phenomena of the growth of roots. Observations 

 were made on this point also, and indicated that 

 the leaf sends out material which favours the 

 growth of roots. This hormone may be the same 

 as that which inhibits the growth of leaf bud.s. 



In the earlier papers a number of experiments 

 were described which indicate that the growth of 

 the buds in the notches of the leaf is dependent 

 on the amount of some material supplied bv the 

 leaf, since the growth is proportional to the mass 

 of the leaf to which the bud is attached. 3 The part 

 played by the direction of the currents in the leaf 

 is also pointed out. The most recent statement of 

 the author's results i is that the production of new 

 shoots by a piece of stem is in direct proportion to 

 the mass of a leaf left attached, and hence to the 

 mass of growth material sent out by the leaf The 

 apex of an intact plant sends inhibiting substances, 

 preventing the buds below from growing. 



A theory of geotropic curvature is put forward, 5 

 in which the growth of the lower side of a stem 

 placed horizontally is explained by the accumula- 

 tion there of growth-promoting substances. The 

 author' 1 appears to hold that the assumption of 

 a "stimulus" due to gravity is unnecessary. Rut 

 there is experimental evidence that the perception 

 of gravity is a separate phenomenon from the pro- 

 duction of the curvature, so that the purely 

 chemical effect by mass action would not be able 

 to explain all the facts. 



It should be noted that the production of in- 

 hibiting substances is a conception arrived at in 

 tr^e later stages of Dr. Loeb's work, so that 

 some of the interpretations given in the earlier 

 papet may require revision. 



It is of interest that as definite a case of corre- 



» "Rules and Mecha 

 tion of Htvcfihvllum ca 



"Chemical Basis of Cm 

 by Equal Masses of Sii 

 lxv., p. ,50. (lOlS.) 



* "The Law Controlling the < 'uantitv and the Ra 

 Proc Nat. Acad Sci., vol. iv., p. 117. (1918.) 



8 " Influence of the Leaf upon Root Formation and Geotropic Curvature 

 of Bryophyllum calycinum and the Possibility of a Hormone 

 Theory of these Processes," Botan. Gazette, vol. Ixiii., p. 15. (1017.) 



8 "The Chemical Basis of Regeneration and Geotropism, \.-ei 

 vol. xlvi., p. 115. (1917.) 



1 of Inhibition and Correlation in the Regenera- 

 HUm" Bolan. Gazette, vol. lx., p. 240. (1915). 

 tion. I. Production of Equal Masses of Shoots 

 Leaves in Bryophyllum calycinum," Md., vol. 



of Regeneration." 



