1. The writer's older experiment, proving that equal masses of isolated sister leaves of Bryophyllum regenerate under equal conditions and in equal time equal masses (in dry weight) of shoots and roots, is confirmed. It is shown that in the dark this regeneration is reduced to a small fraction of that observed in light.
2. The writer's former observation is confirmed, that when a piece of stem inhibits or diminishes the regeneration in a leaf, the dry weight of the stem increases by as much or more than the weight by which the regeneration in the leaf is diminished. It is shown that this is also true when the axillary bud in the stem is removed or when the regeneration occurs in the dark.
3. These facts show that the regeneration of an isolated leaf of Bryophyllum is determined by the mass of material available or formed in the leaf during the experiment and that such a growth does not occur in a leaf connected with a normal plant for the reason that in the latter case the material available or formed in the leaf flows into the stem where it is consumed for normal growth.
4. It is shown that the sap sent out by a leaf in the descending current of a stem is capable of increasing also the rate of growth of shoots in the basal parts of the leaf when the sap has an opportunity to reach the anlagen for such shoots.
5. The fact that a defoliated piece of stem forms normally no shoots in its basal part therefore demands an explanation of the polar character of regeneration which lays no or less emphasis on the chemical difference between ascending and descending sap than does Sachs' theory of specific root- or shoot-forming substances (though such substances may in reality exist), but which uses as a basis the general mass relation as expressed in the first three statements of this summary.
6. It is suggested that the polar character of the regeneration in a stem of Bryophyllum is primarily due to the fact that the descending sap reaches normally only the root-forming tissues at the base of the stem, while the ascending sap reaches normally only the shoot-forming anlagen at the apex of the stem.
7. This suggestion is supported by the fact that when the anlagen for shoots and roots are close together as they are in the notch of a leaf, the sap of the leaf causes the growth of both roots and shoots from the same notch and the influence of the sap of the leaf on this growth increases for both roots and shoots in proportion with the mass of the leaf.