ULTRAVIOLET LIGHT AND THE DEVELOPMENT OF FUCUS EGGS AS AFFECTED BY AUXIN AND pH

¹ From the Department of Biology, Stanford University

1. When Fucus eggs are irradiated unilaterally with suitable moderate doses of 2537 Å, the rhizoids form on the non-irradiated halves of the eggs.

2. The response of populations of eggs is proportional, over a wide range, to the logarithm of the dosage.

3. When moderate doses of unilateral ultraviolet are followed by prolonged exposures to white light from the opposite sides of the eggs, the rhizoids form on the surfaces that were irradiated with ultraviolet. Therefore, the ultraviolet does not determine the position of rhizoid formation merely by causing extensive, general damage and incapacitation of the surface upon which it falls.

4. -indole acetic acid (auxin) has very little effect on the sensitivity of the eggs to moderate doses of ultraviolet at pH 8.0, and none at pH 6.0.

5. -indole acetic acid does not revive eggs in which rhizoid formation and cell division have been retarded or inhibited by heavy doses of 2537 A. Therefore the ultraviolet does not appear to act on the rhizoid formation primarily by affecting or destroying auxin.

6. The sensitivity of the eggs to the polarity-determining effects of moderate unilateral dosages of 2537 A is considerably increased in sea water acidified from pH 8.0 to 6.0.

7. The sensitivity of the eggs to the retarding and inhibiting effects of strong dosages, on the other hand, is considerably decreased in medium acidified to pH 6.0. The two phenomena (directive effect of small doses and inhibitory effect of large doses) thus appear to depend on different actions of the ultraviolet.