attached to it, each notch will give rise to a new plant.[152] (Fig. 17.) We see, therefore, that it does not even require a whole plant to cause inhibition but that we may observe the tyranny of the whole over the parts in a single leaf. The explanation is as follows: When we isolate a leaf, some of the notches will commence to grow into new plants and this growth will arrest the development of the other notches of the leaf in the same way as their development was suppressed by the whole plant. [152] Loeb, J., _Bot. Gazette_, 1915, lx., 249. [Illustration: FIG. 16. Growth of roots and shoots in a few notches of an isolated leaf of _Bryophyllum calycinum_] [Illustration: FIG. 17. If all the notches of a leaf are isolated from each other each notch will give rise to roots and a shoot, but the growth will be less rapid than in Fig. 16. Figs. 16 and 17 were two leaves taken from the same node of a plant.] The explanation is the same; those notches which begin to grow first will attract the flow of substances to themselves, thus preventing the other notches from getting those substances. This idea is supported by the fact that if all the notches are isolated from the leaf each notch will give rise to a slowly growing plant, while if the leaf is not cut into pieces, and a few notches only grow out, their growth is much more rapid. In all these experiments the idea that the “isolation” in itself is responsible for the growth still presents itself. It can be disposed of by the following experiment which never fails. Three leaves of _Bryophyllum calycinum_ are suspended in an atmosphere saturated with water vapour but their tips are submersed in water (Figs. 18, 19, 20). The first leaf, Fig. 20, is entirely separated from its stem, the second leaf, Fig. 19, remains connected with the adjacent piece of stem, and the third leaf, Fig. 18, remains also connected with this piece of stem but the latter still possesses both leaves. The first leaf, Fig. 20, produces new roots and shoots in the submerged part in a few days; the second leaf, Fig. 19, produces no roots or shoots for a long time. This might find its explanation by the assumption that the first leaf, being more isolated than the second, regenerates more quickly. But this explanation becomes untenable owing to the fact that the third leaf, Fig. 18, being less isolated than both (possessing a second leaf in addition to the stem), forms new roots and shoots also more quickly than the second leaf. The phenomena become intelligible in the following way. The fact that in the second leaf shoots and roots are formed very late, if at all, finds its explanation not in the lessened isolation of this leaf, but in the fact that the formation of a new shoot or of a callus in the piece of stem takes place more quickly than the formation of roots and shoots in the notches of a completely isolated leaf. The stem acts therefore as a centre of suction for the flow of substances from the leaf and this prevents