The elements which have entered into the discussion thus far are, first, the flow of substances in preformed channels; second, the existence of general or specific substances required for the growing or regenerating organ. A third element is to be added; namely the “suction” effect upon these substances of a developing organ. Thus we see that if one or a few of the notches in a leaf of _Bryophyllum_ grow out the other notches of the leaf are inhibited from growing. There is enough material present in the leaf for all the notches to grow into shoots as is proved by the fact that all will grow out if they are isolated from each other. This was explained on the assumption that the notches of a whole which happen to develop first, create a flow of these substances from the rest of the leaf to themselves and thus prevent any getting to the other notches. We stated that this is supported by the fact that the few notches growing out in an undivided leaf grow more rapidly than the many shoots growing from each notch of a divided leaf. But why should a growing shoot or a growing point in general produce such a suction? I think this may be possible on the assumption that the consumption of these substances by the growing organs causes a low osmotic pressure of these substances in the growing region and this fall of osmotic potential will act as a cause for the further flow. This brings about the apparent “suction” effect of the growing elements upon the flow of substances. 5. We mentioned that when a piece is cut from a _Planaria_ between pharynx and head a new mouth is formed in the middle. It should also be mentioned that according to Child the piece after regeneration is smaller than it was before.[162] This indicates that material in the old cells has been digested or has undergone hydrolysis in order to furnish the nutritive material for the new head and tail, since the piece cannot take up any food from the outside before a mouth is formed. These phenomena of autodigestion--the process itself will be discussed in the last chapter--seem to occur in many (if not all) phenomena of regeneration. It may be that the collecting of red cells at the end in a Tubularian where regeneration is about to begin has to do with the furnishing of material by self-digestion, since these cells are partly at least destroyed in the process. It is of interest to look for more examples of autodigestion accompanying phenomena of regeneration. [162] Child, C. M., _Senescence and Rejuvenescence_. Chicago, 1915. [Illustration: FIG. 33]