December 7, 1917] 



SCIENCE 



549 



hibitory substance being sent in the direc- 

 tion toward the base by the growing bud, 

 the most apical bud should soon outstrip in 

 growth those situated in the next node be- 

 hind, although the latter had an earlier 

 start. For according to this theory, the 

 most apical buds should be sending sub- 

 stances toward the base which inhibit the 

 growth in the next bud; while the most 

 apical buds receive no such inhibitory sub- 

 stances. The results of the experiment are 

 quite clear. As soon as the petioles at the 

 apex fall off the axillary buds at the apex 

 begin to grow out and soon not only out- 

 strip in size those of the next buds behind 

 but actually retard or stop the growth of 

 the latter. This phenomenon seems intel- 

 ligible only on the assumption that a grow- 

 ing bud sends out substances toward the 

 base of the stem which directly inhibit the 

 growth of the other buds. 



If the inhibition of shoot formation is 

 due to special inhibitory substances it 

 should be possible to show that the inhibi- 

 tion varies quantitatively with the mass of 

 inhibitorj'- substances produced in the 

 growing bud, or with the mass of the latter. 

 While the bud is too small for convenient 

 quantitative experimentation, it can be 

 carried out satisfactorily with the leaf. In 

 a fonner paper the writer had shown that 

 the leaf of Bryophyllum sends out ma- 

 terial toward the base of the stem which 

 favors root formation; and it also seemed 

 possible that the leaf might send out sub- 

 stances in a basal direction which inhibit 

 shoot formation. The sap from the leaf 

 flows in conducting vessels situated in the 

 same half of the stem where the leaf is at- 

 tached. 



When we suspend a stem of Bryophijllum 

 with six or more nodes horizontally, and 

 remove all the leaves except the two in the 



apical node, the stem will form no shoots as 

 long as the leaves are alive, but an abund- 

 ance of roots is produced in the stem. The 

 two leaves, therefore, inhibit all the shoot 

 formation in the buds situated basally 

 from the leaf. When we remove one of the 

 two apical leaves the axillary bud of this 

 leaf will grow out and it will have the same 

 inhibiting effect as the leaf in the previous 

 experiment. We now make the following 

 experiment. 



Twelve long stems from which all leaves 

 except one of the two apical ones have been 

 removed are suspended horizontally, and 

 the free axillary bud opposite the leaf is 

 also cut out. Six stems are suspended with 

 the leaf above, six with the leaf below. 

 There is a striking difference in the two 

 sets. When the leaf is below, shoots will 

 develop either in the two lateral buds of 

 the first node behind the leaf, or on the 

 upper side of the second node behind the 

 leaf. When the leaf is above, no shoots wiU 

 develop in the next node behind the leaf 

 but one shoot may grow in the second node 

 behind the leaf, on the lower side alone. 

 These shoots will develop more slowly than 

 those in the stems whose leaf is on the 

 lower side. 



This is exactly the result which we should 

 expect if the leaf sends out substances in- 

 hibiting shoot formation toward the base 

 of the stem. These substances, being iden- 

 tical with or accompanying the root-form- 

 ing substances, flow on that side of the 

 stem where the leaf is, but have naturally 

 a tendency to flow downward and not to 

 flow upward. Hence, when the leaf is be- 

 low it is possible for shoots to form in some 

 (about 50 per cent.) of the stems in the 

 first node behind the leaf, in which case 

 the buds are on the upper level of the flow- 

 ing sap ; while when the leaf is above it is 

 impossible for the buds in the first node 

 behind the leaf to grow because they are 

 on the lower level of the sap flow from the 



