ハスイモ属はハスイモ elephant’s ear１種からなる(POWO 2025)。ハスイモ（giant elephant ear or Indian taro）の葉は巨大で、1.5 mから3 mにも成長し、葉柄は食用される (Ivancic et al. 2008)。
The genus Leucocasia comprises a single species, Leucocasia gigantea, commonly known as giant elephant ear or Indian taro (POWO 2025). Its leaves are enormous, growing to 1.5 to 3 meters in length, and the petioles are edible (Ivancic et al. 2008).
東南アジア（図の白線の内側）に野生し、日本を含めた地域で栽培されている(POWO 2025)。
It grows wild in Southeast Asia (within the white line on the map) and is cultivated in regions including Japan (POWO 2025).
花序は仏炎苞に覆われ、雌性先熟で、雌花が成熟した１から数日後に雄蕊から花粉が放出される（Ivancic et al. 2008)。雌蕊と雄蕊はそれぞれの成熟期に発熱し、ザセンソウなど、他の温度発生性花序と同じように、花粉媒介昆虫を誘引する（Ivancic et al. 2008)。温度が42度を超える場合もあり、昆虫を追い出す機能もあるのではないかと議論されているが、野外における検証実験が必要である(Ivancic et al. 2008)。
Its inflorescence is enclosed by a spathe, and the plant is protogynous: the pistillate flowers mature first, and pollen is released from the stamens one to several days later (Ivancic et al. 2008). Both the pistils and stamens undergo thermogenesis during their respective maturation phases. As in other thermogenic inflorescences such as Symplocarpus, this heat is thought to attract insect pollinators (Ivancic et al. 2008). Temperatures can exceed 42°C, raising the possibility that the heat may also serve to drive insects away, although this hypothesis requires field testing (Ivancic et al. 2008).
ハスイモの学名はLeucocasia gigantea (Blume)Schottが用いられるが、歴史的に変遷した。Blume (1823)は本種が大きな白い仏炎苞を持つことから、同様に大きな白い仏炎苞を持つ種を含むカラジューム属Caladiumに属すると考え、Caladium giganteum Blumeと命名した。
The scientific name Leucocasia gigantea (Blume) Schott is currently used, though the taxonomy has changed over time. Blume (1823), noting its large white spathe, originally classified the species in the genus Caladium (which includes other species with similar spathes) and named it Caladium giganteum Blume.
その後、Schott (1857)は、カラジューム属Caladiumの種は子房が数室からなり、胚珠が倒生anatropousであるのに対し、本種は子房がほぼ１室subunilocularであり、胚珠が直生orthotropousすることを重視し、本種のみからなるLeucocasiaという新しい属を作り、Leucocasia gigantea (Blume) Schottとした。
Later, Schott (1857) emphasized that while Caladium species typically have multi-locular ovaries and anatropous ovules, this species has a nearly unilocular ovary (subunilocular) and orthotropous ovules. He therefore established a new genus, Leucocasia, consisting only of this species, naming it Leucocasia gigantea (Blume) Schott.
その後、Hooker (1893)は、本種の胚珠が基底胎座basal placentationではなく分散した側膜胎座parietal and scattered placentationであると解釈し、多数の胚珠を付けることなど、Colocasia属に類似していることから、Colocasia gigantea (Blume) Hook.f.とした。
Subsequently, Hooker (1893) interpreted the ovules of this species as having parietal and scattered placentation, rather than basal placentation. Due to this and the large number of ovules, which resemble those of Colocasia, he reclassified the species as Colocasia gigantea (Blume) Hook.f.
しかし、核ゲノムのphytochrome C (PHYC）遺伝子、葉緑体ゲノムのtrnL intron、trnL–trnF intergenic spacer、rpl20–rps12 intergenic spacer, trnK/matK regionの解析から、本種は、クワズイモ属Alocasiaの姉妹種となり、サトイモ属Colocasiaとは単系統にならないことがわかった (Nauheimer et al. 2012)。そこで、現在では、Colocasia gigantea (Blume) Hook.f.ではなく、Leucocasia gigantea (Blume)Schottが用いられる。
However, phylogenetic analyses of the nuclear gene phytochrome C (PHYC) and chloroplast genome regions including the trnL intron, trnL–trnF intergenic spacer, rpl20–rps12 intergenic spacer, and trnK/matK region revealed that this species is a sister group to the genus Alocasia, and not monophyletic with Colocasia (Nauheimer et al. 2012). Thus, Leucocasia gigantea (Blume) Schott is now the accepted name rather than Colocasia gigantea (Blume) Hook.f.
インドクワズイモAlocasia macrorrhizos (L.) G.Donもハスイモに似た厚い大きな葉を形成するが、近縁ではなく(Nauheimer et al. 2012)、この形質は平行進化したと考えられる。
Although Alocasia macrorrhizos (L.) G.Don (commonly known as giant taro) also produces large, thick leaves similar to those of Leucocasia, it is not closely related (Nauheimer et al. 2012), and the shared leaf morphology is considered a result of parallel evolution.
1. 胚珠は多数が側膜胎座 —– 2
1. 胚珠は数個が基底胎座 —– クワズイモ属 Alocasia
2. 植物体は2 mを超えることが多い。仏炎苞は純白。 —– ハスイモ属 Leucocasia
2. 植物体は2 mを超えることは稀。仏炎苞は純白ではない。—– サトイモ属 Colocasia
1. Ovules are many and parietal —– 2
1. Ovules are few and basal —– Alocasia
2. Plants often exceed 2 m high; Limb of spathe pure white —– Leucocasia
2. Plants are usually less than 2 m high; Limb of spathe is not pure white —– Colocasia
引用文献 References
Blume, C.L., 1823. Catalogus van eenige der merkwaardigste zoo in- als uitheemsche gewassen te vinden iv ,s Lands Plantentuin te Buitenzorg/opgemaakt door C.L. Blume. (Reprinted by the Arnold Arboretum, 1946.)
GBIF 2025. Leucocasia gigantea (Blume) Schott in GBIF Secretariat (2023). GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2025-06-14.
Hooker, J.D. 1893. Aroideae. Flora of British India, vol. 6. L. Reeve & Co., London.
Schott, H.W. 1857. Leucocasia, eine Gattung der Colocasinae. Oesterr. Bot. Wochenbl., 7: 33-35.
Ivancic A, Roupsard O, Garcia JQ, Melteras M, Molisale T, Tara S, and Lebot V. 2008. Thermogenesis and flowering biology of Colocasia gigantea, Araceae. J. Plant Res. 121:73–82.
Nauheimer, L., Boyce, P.C., Renner, S.S. 2012. Giant taro and its relatives: A phylogeny of the large genus Alocasia (Araceae) sheds light on Miocene floristic exchange in the Malesian region. Mol. Phylogenet. Evol. 63: 43–51.
POWO (2025). “Plants of the World Online. Facilitated by the Royal Botanic Gardens, Kew. Published on the Internet; https://powo.science.kew.org/. Retrieved 14 June 2025.”