1916] LOEB—BRYOPHYLLUM 205 
sufficiently large Petri dishes containing some water. One leaf 
is cut into as many pieces as there are notches. Every notch gives 
rise to roots and a shoot (fig. 1). The other leaf is left intact and 
in this leaf only a few notches will grow into roots and shoots 
(fig. 2), but the growth is much more rapid in these new shoots than 
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in those of the other leaf. The explanation is simple enough. In 
the intact leaf (fig. 2) the notches which by chance happen to grow 
out first act as if they had a suction effect and caused the current 
of sap to flow away from the other notches, thus preventing their 
growth. This illustrates sufficiently the principle of correlation 
by which the notches of a leaf will not grow out as long as it is 
connected with a healthy plant. In the latter, the suction of the 
