Mat 14, 1915] 



SCIENCE 



713 



that in animals substances circulating in 

 the blood influence growth independently 

 of the central nervous system. In Bryo- 

 phylliim I have recently made some experi- 

 ments which seem to agree with this hu- 

 moral theory of the control of growth. 

 It can be shown in Bryophyllum that if a 

 part a inhibits the growth in a part h, the 

 presence of h favors growth in a. 



We will illustrate this by two experi- 

 ments. When we suspend in the moist air 

 of a closed vessel a stem of Bryophyllum, 

 whose tip, roots and leaves have been re- 

 moved, only the buds in the uppermost 

 node will grow into shoots. The growth of 

 the apical shoots inhibits the growth of the 

 lower buds. But if we isolate a node near 

 the apex and suspend it in the same moist 

 chamber, as a rule no regeneration will 

 occur in this node; only if we leave the 

 lower parts of the stem connected with the 

 apical node can the latter regenerate in 

 moist air. Hence the lower part &, in 

 which regeneration is suppressed by the 

 topmost part a, is necessary or helpful for 

 the regeneration of the top a. 



The same effect can be produced if, in- 

 stead of leaving the node near the apex in 

 connection with the lower pieces of the 

 stem, we leave it in connection with one leaf 

 or part of one leaf. In this case also 

 growth of the bud will occur in the moist 

 air. As we have already stated, the leaf is 

 inhibited from forming new shoots in its 

 notches through the connection with the 

 stem. Hence the stem which inhibits the 

 growth of shoots in the leaves is helped by 

 the leaf in its own regeneration. 



This seems to agree at first sight with 

 the idea first suggested by Sachs that the 

 specific shoot-forming substances do not 

 exist in sufficient quantity in the topmost 

 part of the stem and that they must be 

 supplied to this piece either by a leaf or by 

 a larger piece of stem. And on the same 



principle might be explained the inhibition 

 of the top piece upon the regeneration of 

 the lower nodes. To this assumption the 

 simple objection is possible that a long 

 stem contains material enough to form a 

 dozen shoots or more, as can be shown if the 

 stem is cut into shorter pieces. Each of the 

 lower nodes will in this case form two new 

 shoots. Yet the formation of two shoots 

 at the apical node will prevent the forma- 

 tion of shoots at the lower nodes, although 

 there is enough material to form shoots in 

 every node. 



It can be shown that the upper nodes 

 if isolated will promptly form shoots if put 

 into a thin layer of water. Hence the 

 presence of a leaf or of the greater part 

 of a stem enables the upper node to form 

 shoots in moist air either by supplying it 

 with the necessary amount of water or by 

 establishing a flow of material. Where we 

 have a closed circulatory system as in ani- 

 mals we know that the heart action can only 

 maintain a circulation if the blood vessels 

 are filled with blood. The writer is not 

 sufficiently familiar with the circulation 

 in plants, but botanists do not assume the 

 existence of a closed circulatory system. 

 But, however this may be, the presence of 

 a sufficient quantity of water seems to be 

 the prerequisite for a constant flow of sub- 

 stances in the conducting vessels. If we 

 assume that the anatomy of the conducting 

 vessels determines a flow of substances to 

 the apex and second that the buds in that 

 region hold all or practically all the forma- 

 tive or specific material which induces 

 growth, the inhibition of growth in the 

 lower buds becomes clear. 



Hence we are inclined to explain both the 

 inhibiting effect of an organ a upon the re- 

 generation in h as well as the accelerating 

 effect of & upon a, from the following three 

 factors: first, the peculiarities of the anat- 

 omy of the conducting vessels in the plant ; 



