Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Asterids
Order: Apiales
Family: Apiaceae
Genus: Apium
Species: A. nodiflorum
Binomial name: Apium nodiflorum
Synonyms: Helosciadium nodiflorum, Apium nodiflorum var. vulgare, Helodium nodiflorum, Meum nodiflorum, Pimpinella nodiflora, Selinum nodiflorum, Seseli nodiflorum, Sium nodiflorum.
Common names: Fools watercress, Water celery

Apium nodiflorum is a sprawling, emergent, perennial, freshwater aquatic, wetland weed found in ditches or on the edges of slow-moving streams, and in marshy grasslands and anthropogenic habitats such as gravel pits.
It is native to western Europe. and is found throughout the North Island and the north and west of the South Island of New Zealand.
It has prostrate and ascending hollow, grooved, striate, stems that can grow over a metre in height.
It has bright, glossy, green pinnate leaves arranged in pairs up each stem. They have a vague resemblance to those of watercress.
From November to February it develops short-stalked umbels (>4 cm diameter) with small (2 mm) white 5 petalled flowers. The ovoid seed cases are ribbed and are about 2mm long.

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The surface of a leaf.
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The underside of a leaf.
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Thanks to Wikipedia for text and information: