water lily
General term for any plant of the genus Nymphaea.
池に睡蓮が咲いている。
Water lilies are blooming in the pond.
pygmy waterlily
Specifically Nymphaea tetragona, a small species of water lily. Often referred to as ヒツジグサ in Japanese.
睡蓮の中でもヒツジグサは特に小さい。
Among water lilies, the pygmy waterlily is especially small.
Standard kanji spelling for water lily.
Katakana spelling common in botanical contexts or when emphasizing the plant name.
Irregular kanji usage; may be encountered but not standard.
From Chinese 睡蓮 (shuìlián), literally 'sleep lotus', referring to the way the flowers close at night.