The Nature of the Directive Influence of Gravity on the Arrangement of Organs in Regeneration

1. When leaves of Bryophyllum calycinum are suspended in moist air in a vertical plane and sidewise, roots and shoots are formed exclusively or predominate in the notches on the lower side of the leaves. When pieces of stems of the same plant are suspended horizontally in moist air, roots develop on the lower side of the stem, with the exception of the extreme basal end where they may develop on both sides. 2. The writer has suggested in a preceding paper that this directive influence of gravity on the arrangement of the regenerating organs may be due to the combination of two factors. The first factor is gravity, which causes a slightly greater collection of sap on the lower side of these organs, and as a consequence roots develop a little more quickly on the lower than on the upper side. The second factor is of an inhibitory character inasmuch as quite generally organs which grow out first, or which grow quickly, have a tendency to retard or inhibit the growth of similar organs in other places. 3. The writer was able to prove the action of this inhibitory factor by cutting off the lower edges of leaves suspended sidewise in a vertical plane or the lower halves of stems suspended in a horizontal plane (in moist air). In this case roots developed as abundantly on the upper side of these organs as they otherwise would have developed on the lower side. 4. It was, however, still necessary to prove the idea that gravity causes sap to collect in larger quantity in the lower parts of organs. This gap is filled by the present paper in which it is shown, first, that in the leaves suspended in moist air a red pigment is formed which has a tendency to collect gradually in the lowest parts of the leaf when the latter is suspended in a vertical plane. This red pigment serves as an indicator for the distribution of sap in the leaf and thus shows directly the tendency of the sap to collect in greater abundance on the lower edge of a leaf suspended in a vertical plane. Second, it is shown that when leaves or stems of Bryophyllum are suspended, in the way described, under water instead of in moist air, roots develop on the upper side as well as on the lower side. The directive effect of gravity upon the arrangement of organs disappears in this case since the abundance of the outside water makes the effect of a slight difference in the distribution of sap between the upper and lower side a negligible factor. Third, it is shown that the dry weight of the lower half of leaves suspended sidewise for several weeks in moist air in a vertical plane is greater than that of the upper half when roots and shoots are formed on the lower side only. This indicates that material from the upper half flows into the growing organs of the lower half. No such difference between upper and lower half of leaf is found when the leaves are suspended in the same way in water and roots and shoots are formed on both sides of the leaf. 5. It is shown that when a leaf connected with a piece of stem is suspended in moist air the red pigment goes into the stem instead of collecting in the lower part of the leaf, thus supporting the view expressed in a preceding paper that the inhibitory action of the stem on the root and shoot formation in a leaf of Bryophyllum is due to the fact that the material available in the leaf for organ formation is naturally sent into the stem.