50 ROSS AIKEN GORTNER AND J. ARTHUR HARRIS 
place below this, but always above one of the more proximal nodes, 
never in the more median or distal regions of the internode.^ 
In some cases abscission above a node more proximal than the one 
immediately below the cut seems to follow an operation in which the 
internode was cut off very close indeed to the node, but this may also 
follow the sectioning of the internode at any point. When the whole 
of the internode below the one which was injured is cut off by an 
abscission zone, the axillary bud of the distal node is almost always, 
but not invariably, abortive. Whether this is the cause of, or the 
consequence of, the more proximal abscission than is normal we cannot 
assert. 
6. After a first abscission above the subtending node another may 
later occur above some lower node. Possibly in this case the bud of 
the subtending node becomes abortive after the first abscission, but 
we can make no final statement on this point. 
7. The separation of the segments of internode or internodes seems 
to be brought about by the disorganization of the tissues in a (gener- 
ally) definite zone across the axis. Whether the cell walls are them- 
selves broken down, or whether the cells are only separated, w^e have 
not been able to determine with certainty. The evidence furnished by 
the few microtome sections which we have made seems to favor the 
first hypothesis. When the section of internode falls the two surfaces 
are moist but usually very firm and smooth. Occasionally, however, 
the region in which the tissue is disorganized is more extensive and 
more irregular. In rare instances a considerable portion of an inter- 
node becomes soft and pulpy, the epidermis only remaining firm; 
possibly this is due to bacterial infection rather than to the processes 
by which the formation of the abscission zone is normally brought about. 
8. In a number of experiments with our common native form, 
/. fulva, the cut end of the twig simply dried up. Abscission of the 
axis was never observed. 
9. Any information concerning other species of plants in which 
similar phenomena occur will be very welcome, since we hope later 
to treat the subject more comprehensively. 
Station for Experimental Evolution, 
Cold Spring Harbor, Long Island, N. Y. 
^ We have seen one apparent exception to this rule, but the internode was in this 
case very short. 
