EFFECT OF PLANT HORMONES ON ULVA

Abstract

1. Bacteria-free Ulva lactuca, in sea water media enriched with vitamins, grows as atypical, short, filamentous germlings which do not develop into a foliaceous thallus. These filaments reach a few millimeters, then bleach, leaving a few islands of intensely green cells which, upon transfer to new media, produce new germlings. 2. The initiation of new germlings from these green islands and the length of the atypical filaments are increased by the combination of kinetin + indolacetic acid; adenine and indolacetic acid seem antagonistic. Conspicuous elongation of the filaments is promoted by the addition of gibberellins to the kinetin-indolacetic acid combination. 3. A normal flat blade was obtained so far only with adenine + kinetin. 4. The responses depend both on the interaction and concentrations of these morphogenetic agents: indolacetic acid is effective only in a very narrow range of concentrations and a blade is produced only with relatively high concentrations of kinetin. 5. The rhizoids of Ulva and the cells of the green islands can produce directly new germlings and are far more resistant to unfavorable conditions than the other cells of the thallus which can originate zoospores or gametes. The morphogenetic and ecological significance of the resistant cells is discussed. 6. These responses of a relatively simply organized sea weed to plant hormones link even more tightly the green algae to the higher land plants. 7. The variety of evolutionary steps toward increased morphological complexity in the algae suggests that whole organisms, because of their relative morphological simplicity, may be valuable experimental material for studying the mode of action of plant hormones.