In two former publications' it was shown that while the stem of Bryophyllum calycinum prevents or retards the development of roots and shoots in the notches of a leaf, conversely the leaf accelerates the development of roots and shoots in a stem; since in a stem deprived of all leaves the roots and shoots develop later and grow more slowly than if a leaf is left on the stem. The two phenomena found a common explanation in the assumption that the leaf furnishes substances to the stem which accelerate the organ formation in the latter, while if these substances are not "sucked away" from the leaf by the stem they will accelerate the growth of roots and shoots in the notches of the leaf. These substances may be water or solutes. In these experiments it was noticed that the leaf has also an accelerating effect upon the geotropic curvature of the stem. When stems of Bryophyllum are suspended horizontally by 2 threads in a jar saturated with water vapor, they will bend, becoming convex on the lower, and concave on the upper side (fig. I), and this bending continues until finally the stems assume the shape of a U. This geotropic bending is a slow process when the stem contains no leaf, but is considerably accelerated if a leaf is left on the stem (fig. I). The position of the leaf has a great influence, not only on the velocity of the geotropic bending and the region of the stem in which it occurs, but also upon the formation of organs in the stem. The description of this influence and of the apparently close connection between the two groups of phenomena will form the subject of this paper.
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Jacques Loeb (1917) studied this question.