The possible rôle of constant bioelectric currents in growth.

Abstract

In a previous note¹ it was stated that the threshold density of an electric current necessary for inhibition of growth in the ends of Obelia internodes turned toward the cathode was very nearly equal to 66 microamperes per square millimeter.

This threshold inhibition is reversible.² For orientation of the axis of growth a threshold current density of only 6 to 13 microamperes per square millimeter of cross section is necessary. This is therefore only about one-tenth as great as the current density threshold for inhibition. We may translate this result into its equivalent of threshold of applied difference of electric potential.

The living tissue of the stem and pieces of stem in Obelia have also been shown to be the seat of a relatively constant electric potential.³ More recently this potential has been measured by the use of a suitable type of electrode.⁴ The results show that a difference of potential exists between the inside and outside of the ecto-endoderm layer. The outside surface of the ectoderm being electro-positive to the inside surface of the endoderm. Similar potential differences are well known to occur in the skin of the frog, intestine and some other epithelial structures. The magnitude of the inherent potential in the ecto-endoderm of Obelia is greater in the apical growing point of the main stem of the colony than in the middle, and somewhat lower regions of the same stem of the same stem. The same statement applies to growing branches of the colony. The result of this difference in the magnitude of the electric potentials across the ecto-endoderm in apical and more basal parts of the stem, is that when the outer surfaces of the apical and basal regions of the stem are connected to a galvanometer, a current flows in the galvanometer circuit from apical to basal parts of the isolated stem.