Bento boxes, invented in Edo times, hold many morsels of different tastes in a decorative arrangement. In order to keep these morsels - and therefore their tastes - apart, a variety of separating materials is used. These days, most often green plastic strips cut to look like grass are used for that. However, the traditional use of certain leaves, cut to release the plant's own microbicides, are often better for keeping the food separated and fresh. The type of leaves traditionally used differ from area to area. With Aspidistra elatior (a member of the lily family) being used in Kansai (including Kyoto) and sasanoha (leaves of bamboo grass) being used in Edo (Tokyo).
[Source: http://search.japantimes.co.jp/cgi-bin/ek20080115wh.html]
[Source: http://search.japantimes.co.jp/cgi-bin/ek20080115wh.html]