THE INHIBITORY ACTION OF APICAL LEAVES 107 



The following objection can be raised to these experiments; 

 namely, that the descending sap sent out by the leaf in crossing 

 over to the other side of the stem underwent a change, consisting 

 in the loss of the inhibitory substance. To test the possibility 

 of such a modification of the sap in crossing over from one side 

 of the stem to the other, the following experiment was made: 



Stems with one leaf left at the apex were split lengthwise at 

 some distance beneath the leaf, until near the base, which dipped 

 into water (Fig. 83, a). The apical bud opposite the leaf was 

 removed as indicated diagrammatically in the figure. In this 

 case the sap from the leaf had to cross over to the opposite side 

 and then to flow down the stem on the side opposite the leaf and 

 on this side the first roots developed at the base, spreading finally, 

 however, all around the base. No shoot developed on that side 

 (Fig. 83, a), showing that in crossing over the sap had not lost its 

 inhibitory power. A shoot developed, however, on the same side 

 where the leaf is, at the apex of the split part of the stem (Fig. 

 83, a). Now the question arose, whether or not the descending 

 sap of the leaf contributed to the growth of this shoot. This 

 turned out to be true as the following quantitative experiment 

 shows: 



Five stems without leaves were split lengthwise down to near 

 the base, as shown in Fig. 83, h, each spht half producing a shoot 

 at the apex. Six stems of almost the same mass but with one 

 leaf left at the apex were spht as indicated (Fig. 83, a). The 

 latter stems produced one shoot at the apex of the split half of 

 the stem as indicated in Fig. 83, a, and occasionally a second 

 smaller shoot in the node below. Now the total dry weight of 

 shoots produced by the six stems in a was 1.557 grams, while the 

 total dry weight of shoots produced by the five stems in h, without 

 leaves, was only 0.668 gram. The dry weight of the six apical 

 leaves was 2.063 grams, about the same as that of the stems. 

 This removes all doubt that the descending sap from the apical 

 leaf contributed to the growth of the shoots below the leaf in a. 

 In this case, the descending sap had to travel the whole length of 

 the stem on the side opposite the leaf (Fig. 83, a), and had then 

 to ascend again to the apex of the split part of the stem. The 

 exact figures are given in Table XXIII. The mass of regenerated 

 shoots is so great because the experiment lasted longer than usual; 

 namely, over two months (Jan. 9 to Mar. 16). 



