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1920] CHILD & BELLAMY— BRYOPHYLLUM 265 



nutritive substances available must depend upon the size of the 

 leaf, but such relation tells us nothing concerning the factors 

 which initiate the development. 



On the other hand, the assumption of the transportation of 

 inhibiting substances, made in Loeb's later papers, also involves 

 certain difficulties. In the first place, each part which produces 

 such substance or substances must be immune to the action of 

 the substance which it produces, since it is not inhibited by it, 

 yet in the case of growing tips the substance produced by one 

 growing tip inhibits other tips. This presupposes a remarkable 

 specificity of action on the one hand a^d absence of specificity on 

 the other, and it is difficult to conceive how the hypothetical sub- 

 stance could possess the properties required. Certain assumptions 

 concerning the direction of flow of the inhibiting substances also 

 have no basis in fact and do not agree well with the facts at hand. 

 Certain other objections to the assumption of inhibiting substances 

 scarcely require discussion in view of the work of various botanists 

 and the experiments just described. 



In an early paper Loeb (14, pp. 251-253) endeavored to show 

 that isolation is not the initiating factor in the outgrowth of buds 

 on the leaf of Bryophyllum, and described three experiments to 

 prove his point (see his Jigs. 1,2,3). In the first a leaf partially 

 submerged is completely separated from the stem; in the second 

 it remains attached to a piece of stem cut off above and below the 

 node and the opposite leaf is removed, but its axillary bud remains; 

 and in the third the opposite leaf also remains. In the first experi- 

 ment and in the third the submerged buds develop, in the second 

 they do not, but the axillary bud of the opposite side develops in 

 the absence of its leaf. Loeb maintains that the leaf in the second 

 experiment is more isolated than in the third, but its buds do not 

 grow out, therefore isolation cannot be the factor determining the 

 development of the buds. This conclusion is incorrect and based 

 upon a misconception of isolation. Actually the leaf of the 

 second experiment is less isolated than in the first and third, 

 because in this experiment the axillary bud of the opposite side 

 develops and inhibits the leaf buds. If this growing tip is removed, 

 the buds of the leaf will develop. In the third experiment the 



