Polarization of fucoid eggs by steady electrical fields

Pelvetia eggs were exposed to steady electric fields from 5 hr after fertilization until their rhizoids began to grow out some 6 to 10 hr later. Eleven batches of eggs responded by initiating rhizoids towards the positive electrode; two batches responded by growing towards the negative electrode; and three grew towards the negative one in small fields and towards the positive one in higher fields. Polarization, defined as the average cosine of the outgrowth directions, was proportional to field strength up to polarization values of 50% for the positive responses and 75% for the negative ones. A voltage drop of 6 mV/cell induced 10% polarization in the positively galvanotropic batches, while 3 mV/cell did this in the negative ones. We reason that both responses are mediated by faster calcium entry at the future growth point. It is supposed to be faster there in positively galvanotropic eggs because the membrane potential, hence the driving force, is highest; in negatively galvanotropic eggs because depolarization induces an overbalancing increase in calcium permeability there.