1918] LOEB—CORRELATION I7I 
rate of growth of a notch is linked a greater inhibiting power upon 
the growth of the other notches, inasmuch as the flow of sap is 
directed toward a rapidly growing notch. The leaf in fig. 15 was 
then taken out of water and suspended in air on February 4. No 
new notches grew out, as was to be expected. The rapidly growing 
original shoot attracted all the sap available. 
A few roots started in some of the notches, 
but shriveled up almost as soon as they were 
formed (fig. 16). The results of this experi- 
ment are as constant as those of the previously 
mentioned experiment. 
These observations thus give us a rather 
clear view of the mechanism of correlation in 
an isolated leaf. In order to start the growth 
of a notch it is necessary that a stream of 
water should reach the notch. This will not 
happen as long as the leaf is part of a stem. 
Only when the leaf is old, ready to drop from 
the plant, do we notice occasionally that a 
shoot may form in the notches of a leaf while it 
is still attached to the plant, but this is rare. 
We can start the growth of notches at will, Fic. 16.—Same leaf 
however, when the leaf is cut off. In that case rs . ; soak i 
that notch or those notches will grow first van es a 
which happen to receive the greatest water pended in moist air; 
supply (from within or without). Those which ‘apidly growing old 
begin to grow more rapidly than the rest will fo ther growth in 
automatically cause a current of sap toward other notches. 
themselves, in a way not yet understood. 
They thereby inhibit or retard the growth in the other notches. 
This inhibition can be overcome at any time by supplying more 
water to an inhibited notch from without, whereby we acceler- 
ate the rate of chemical reactions in this notch, which in turn 
will now cause a flow of sap toward itself, but we can also 
increase the flow of sap to certain notches from within. The 
writer’s former observations have shown that the sap in the 
leaf can flow around a corner, a fact which suggests the existence of 
