We will now show that the assumption of the existence of such "organ-forming" substances (which may or may not be specific) and of their flow in definite channels explains the inhibitory influence of the whole on the parts as well as the unbridled regeneration of the isolated parts.
2. We have seen that the resting egg can be aroused to development and growth by substances contained in a spermatozoön or by certain other substances mentioned in the preceding chapter. We will assume that plants contain a large number of cells or buds which are comparable to the resting egg cell, but which can be aroused to action by certain substances circulating in the sap; and that the same is effected for animal cells by substances in the blood. In plants the cells which can be aroused to new growth have very often a rather definite location while in lower animals they are more ubiquitous. For experimental purposes organisms where these buds have a definite location are more favourable, since we are better able to study the mechanism underlying the process of activation and inhibition (correlation). When a leaf of the plant _Bryophyllum calycinum_ is cut off and put on moist sand or into water or even into air saturated with water vapour, new plants will arise from notches of the leaf. This is the usual way of propagating the plant and in no other part of the leaf except the notches will new plants arise. These notches therefore contain cells comparable to seeds or to unfertilized eggs or to the mesenchyme cells which give rise to legs in the tadpole of the frog. The question arises: Why do notches in the leaf never begin to grow while the leaf is attached to an intact plant, and why do they grow when the leaf is isolated? To this we are inclined to give an answer in the sense of Bonnet, Sachs, de Vries, and Goebel, namely that the flow of (specific?) substances in the plant determines when and where dormant buds or anlagen shall begin to grow. Such substances may originate or may be present in the leaf; but as long as it is connected with a normal plant they will be carried by the circulation to the growing points of the stem and of the roots and they cannot reach the notches; while when we detach the leaf, either a new distribution or a new flow of liquids will be established whereby the substances reach some of the notches; and in these notches new roots and a new shoot will be formed. When we cut off a leaf and put it into moist air, not all but only a few of the notches will, as a rule, grow out (Fig. 16); but when we isolate each notch leaving as much of the rest of the leaf as possible 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.