Key to species of Sargassum

1 Primary branches ("stem") smooth, without projections or spines 2
1

Primary branches ("stem") with spinous projections

S. polyphyllum
2 Primary branches flattened at base; receptacles spiney, vesicles ("floats") on flattened petioles sometimes common S. echinocarpum
2 Primary branches flattened or compressed upwards from base to second or third secondary branch; receptacles smooth and finger-like; vesicles ("floats") on cylindrical petioles S. obtusifolium

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