Abstract
The formation of zoospores or gametes in the green alga Ulva mutabilis Føyn is shown to be blocked by some substance(s) which can be extracted into fresh growth medium from a suspension of living thallus fragments. The inhibiting power of thallus extract is greatly reduced by heat treatment, but heat-stable components may also be present since active extracts can be produced by boiling algal thalli in growth medium. Separation of the extract on a Sephadex G25 column gave two peaks of inhibitory activity which corresponded with the fraction's content of high molecular carbohydrate (probably ulvin). Various mono- and disaccharides were inactive. In order to block meiotic zoospore formation completely, thallus extracts must be applied at least 6-7 h prior to the cells' visible entry into the meiotic prophase.
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