550 



SCIENCE 



[N. S. Vol. XLVI. No. 1197 



leaf. The bud on the lower side of the sec- 

 ond node behind the leaf (when the latter 

 is on the upper side of the stem) is outside 

 the sap flow and hence it may develop. 



"When we work with a large apical leaf 

 attached to a short stem (the free apical 

 bud opposite the leaf is always removed in 

 these experiments) containing only two 

 nodes behind the leaf, everything is as de- 

 scribed for long stems. When, however, 

 the piece of stem behind the leaf is smaller, 

 containing only one node, no shoot can grow 

 on this stem even when the leaf is below. 

 The m'ass of inhibitory substance sent out 

 by a large leaf will flood the buds in this 

 node with inhibiting material. Occasion- 

 ally a bud starts to grow but stops before 

 a leaflet has time to unfold. Such a stem 

 will form an abundance of roots at the 

 base. If, however, we reduce the size of 

 the apical leaf by cutting away nine tenths 

 of its mass, most or practically all the 

 stems will form shoots in the node behind 

 the leaf; but roots in such stems either do 

 not develop at all or only with long delay. 



The leaf, therefore, sends substances to 

 the basal part of the stem which inhibit 

 shoot formation and favor root formation, 

 and the mass of these inhibitory substances 

 decreases with the mass of the leaf, and ap- 

 parently parallel with the mass of root- 

 forming substances sent to the base of the 

 stem. 



Another experiment is equally instruc- 

 tive. We have seen that when long stems 

 having all but one apical leaf removed 

 (and the opposite free apical bud also re- 

 moved) are suspended horizontally, with 

 the leaf above, no shoot will form on the 

 upper side of the stem. Wlien we reduce 

 the size of the leaf sufSciently this inhibi- 

 tion ceases. 



Again the objection might be raised that 

 the inhibiting effect of the leaf on shoot 

 formation in the region behind the leaf is 

 due not to an inhibitory substance being 



sent out by the leaf but by nutritive sub- 

 stances needed for the growth of shoots 

 being sent into the leaf by the stem. This 

 is highly improbable not only on the basis 

 of our knowledge of these processes but 

 also on account of the following fact. 

 When we cut off a leaf without its petiole, 

 leaving the latter in connection with the 

 stem, the petiole will dry out and fall off in 

 a week or less. If, however, the petiole is 

 detached from the stem but left attached 

 to a leaf, it will not wilt, but remain fresh 

 and green as long as the leaf is alive, which 

 may be many months. This shows that nu- 

 tritive material is furnished by the leaf to 

 the stem, and not vice versa. 



While these experiments show that the 

 inhibiting influence of an apical bud on the 

 growth of the more basal buds is due to 

 one or more inhibitory substances being 

 sent toward the basal end of the stem, the 

 other main fact of polarity remains unex- 

 plained ; namely, how it happens that the 

 most apical bud grows out first. The 

 writer is inclined to offer the following 

 suggestion: In the normal plant, the sub- 

 stances inhibiting shoot formation are con- 

 stantly flowing from the growing region 

 toward the root of the plant. When we cut 

 out a piece of stem and remove the leaves 

 these substances will at first exist in every 

 node, but will continue to flow toward the 

 base. Hence the most apical node will be 

 the first one to be free from these inhibitory 

 substances and the bud or buds situated 

 here can now begin to grow out. As soon 

 as they grow out they will maintain a con- 

 stant flow of inhibitory substances toward 

 the base which will suppress the growth of 

 buds in the more basal part of the stem. 



The experiments, therefore, seem to prove 

 that axial polarity in the regeneration of 

 a stem is due to the fact that the apical bud 



