1918] LOEB—CORRELATION 167 
later. Two of the shoots in the notches in the middle of the leaf 
have grown into shoots, and in these notches the roots have con- 
tinued to grow; while the roots formed in the other notches have 
shriveled up and no new shoots have grown out. 
Fic. 11 Fic. 12 
Fics. 11, 12,—Same leaf suspended in moist air, in fig. 11 after 18 days, in fig. 12 
after 28 days; at first all notches in middle of leaf form roots and in some of them 
shoots begin to develop (fig. 11); later (fig. 12) only two of these shoots in middle of 
leaf grow, while roots in other notches not only ceased to grow but are shriveled up; 
proves inhibiting effect of most rapidly growing notches on others. 
From this observation, which is typical and which has been 
verified many times, we are inclined to draw the following con- 
clusion. As long as the leaf is part of the normal plant, its sap 
flows into the stem of the plant and the notches cannot grow out. 
When the leaf is separated from the plant and suspended in moist 
air, this flow ceases and the material carried in the form of sap 
