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.

Fig. 16. Growth of roots and shoots in a few notches of an isolated leaf of Bryophyllum calycinum   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.

Fig. 18      Fig. 19     Fig. 20

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 or retards the forma­tion of roots and shoots in the notches. In the isolated leaf of Bryophyllum calycinum no callus forma­tion takes place and hence no flow of the sap away from the leaf will occur. This will allow one or more of the notch buds of this leaf to grow out and then a flow will be established towards these growing buds.

In the third specimen, Fig. 18, the presence of two leaves suppresses or, as a rule, retards the growth of a shoot on the stem and possibly also the flow from one leaf may block to some extent the flow from the opposite leaf if the piece of stem is very short. This puts the leaves in a condi­tion not as good as that in leaf Fig. 20, but better than in leaf Fig. 19.[153]

In the normal plant the buds in the notches of the leaf remain dormant since the flow of the “stimulating” substances takes place towards the tips of the stem and root, and because these substances are retained there in excess. This is probably the real basis of the mysterious dominance of the “whole” over its “parts” or of the anlagen of the tip of the stem over those farther below. When a piece of the stem of Bryophyllum is cut off and its leaves are removed, the two apical buds will grow out first. This “dominance” finds its explana­tion probably in the anatomical structure and the mechanism of sap flow which tend to bring the “stimulating” substances first to the anlagen in the tip. In Laminaria Setchell has been able to show directly that regenera­tion always starts from that tissue which conducts nutritive material.

When we cut out a piece of a stem of Bryophyllum, and remove all the leaves, new shoots will be formed from the two apical buds of the stem, and roots will arise from the most basal nodes; provided that the stem is suspended in air saturated with water vapour. The growth in such a stem deprived of all leaves is slow. If we remove all the leaves on such a piece of stem except the two at the apical end, the stem will form only roots, but these will develop much more rapidly than on a stem without leaves. If we remove all the leaves except the two at the basal end, the stem will only form shoots (at the apical end) but these will develop much more rapidly than in a leafless stem. Hence the leaves accelerate the growth of roots towards the basal end and inhibit it towards the apical end; and they favour the growth of shoots towards the apical end and inhibit it in the nodes located nearer the base.

We thus see that while the stem inhibits the growth of the leaves connected with it, the latter accelerate the growth in the stem. Both facts can probably be explained on the same basis; namely, on the assump­tion that it is the flow of substances from the leaf to the stem which inhibits the growth of the notches and accelerates the growth of the buds in the stem. On this assump­tion it would also follow that the leaves send root-forming substances towards the basal and shoot-forming substances towards the apex of the stem. It also seems to follow from recent as yet unpublished experi­ments by the writer that the root-forming substances are associated or identical with the substances which cause geotropic curvature in the stem.