Viola grypoceras A.Gray var. exilis (Miq.) Nakai

Family: Violaceae
Japanese: コタチツボスミレ (小立坪菫) [Kotachitsubo Sumeré]

This violet has been found to be dwarfed, but not genetically isolated from lowland counterparts. [2009 Kyoto University Field Report, http://gcoe.biol.sci.kyoto-u.ac.jp/gcoe/jpn/report/ ] I don't really remember a difference in size, but maybe that's because the brush where they live in my neighborhood gets mowed.

References:
屋久島の植物 川原勝征,初島住彦(監修),2001：南方新社
米倉浩司・梶田忠 (2003-)　「BG Plants 和名－学名インデックス」（YList），http://ylist.info（ 2019年8月14日）
2009 Kyoto University Field Report, http://gcoe.biol.sci.kyoto-u.ac.jp/gcoe/jpn/report/