Ludwigia hyssopifolia
[Narrow-leaved Malayan Willow-herb, Seedbox]

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Close-up view:

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This plant has small yellow flowers, with petals about 2 to 4 mm long. There is a similar looking plant with larger flowers, named L. octovalvis, having petals of about 1 to 2 cm long. Each flower should have 4 petals. The flower in the left picture below has 5 petals, which is considered rare. This was the only 5-petal flower I saw on that plant while the rest of the flowers on the same plant have 4 petals. The right picture below shows L. hyssopifolia growing together with L. octovalvis. The plant with long, narrow leaves is L. octovalvis.

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