548 



SCIENCE 



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



be considerably retarded. None of the 

 buds in the other nodes will grow out. 

 Boots will grow chiefly on the under side 

 of the stem, but in the last node and at the 

 cut end they may form on the upper side as 

 well as on the lower side of the stem. 



Experiment II. is the same as Experi- 

 ment I., except that the upper apical bud 

 is cut out. In this case the lower apical 

 bud will grow rapidly, but in addition one 

 or both of the buds of the node next to the 

 apical will grow out. These buds never 

 grow out when the upper apical bud is pre- 

 served and healthy. 



Experiment III. is the same as the pre- 

 vious experiment except that the lower 

 apical bud is removed, while the upper onb 

 is preserved. In this case, the upper apical 

 bud will grow out, but none of the others. 



It follows from these experiments that the 

 upper apical bud inhibits or retards the 

 growth of the lower apical bud as well as 

 that of the rest of the buds; while the 

 lower apical bud can not suppress the 

 growth of the buds in the node behind. 

 The writer has repeated these experiments 

 in many modifications, among which those 

 on longitudinally split stems are the most 

 striking. The results were uniform. 



All these observations are intelligible if 

 we assiune that a bud when it begins to 

 grow produces and sends out inhibitory 

 substances toward the base of the stem. 

 These substances flow in the conducting 

 vessels in the same half of the stem where 

 the bud lies ; when one apical bud is above 

 and one below, the two buds in the next 

 node are in a lateral position between the 

 upper and lower half of the stem. Hence 

 the inhibitory substances sent out by the 

 upper apical bud can reach the two buds 

 in the next node behind and inhibit their 

 growth, since these buds lie directly below 

 or on the lower level of the conducting ves- 

 sels from the upper apical bud; while in- 

 hibitory substances sent out by the lower 



apical bud can not reach the buds in the 

 node behind in large quantity, since these 

 buds are on the upper level or slightly 

 above these conducting vessels. Wlien the 

 two lateral buds grow out they wiU inhibit 

 the growth of all the buds behind, each bud 

 covering a territory of one half stem. 



The alternative hypothesis assumes that 

 since the apical bud is the first to grow out 

 it will absorb all the shoot-forming ma- 

 terial.^ If we assume that the shoot-form- 

 ing material has a tendency to rise this 

 hypothesis may explain the facts also. But 

 the following experiment, which seems 

 crucial, decides in favor of the other as- 

 sumption. 



A piece of stem containing a number of 

 nodes is suspended horizontally, as in the 

 previous experiments, with the two apical 

 buds in a vertical line. All the leaves are 

 removed with the exception of those at the 

 apical node. Here the petioles of the leaves 

 are left attached to the stem, the leaves 

 having been cut off. The petioles will wilt 

 in a week or ten days, but until then will 

 prevent or retard the growth of the apical 

 buds in their axils. The buds in the next 

 node will begin to grow out and as soon as 

 the petioles have fallen off the apical buds 

 will also begin to grow. 



The next step is decisive for testing the 

 two hypotheses. If the inhibiting effect of 

 the apical buds on the more basal buds is 

 due to the fact that the buds which grow 

 out first attract all the material from the 

 basal part of the stem, the buds in the node 

 behind the apical one, which grew out first, 

 should continue to outstrip in growth the 

 apical buds which began to grow out later. 

 But if the inhibiting effect is due to an in- 



1 This form of inhibition exists apparently in 

 the leaf where the shoots which grow out first 

 prevent other notches in the leaf from giving rise 

 to shoots by absorbing the material needed for 

 shoot formation. Science, 1917, XLV., 436; 

 XLVI., 115; Bot. Gas., in print. 



