Curio radicans leaf window slit

The leaf windows of Curio radicans are translucent surface bands from base to tip of each leaf body, coloured differently. This window constitutes a gap in the normal leaf wall tissue for allowing sunlight to penetrate.

Leaf windows or fenestration facilitating enhanced photosynthesis occur in many kinds of succulents, and take many shapes, well-known in for instance in Lithops and some Conophytum species.

Light passes through the clear jelly-like cells storing water inside the leaf body, reaching chlorophyll in cells deep inside. Photosynthesis happens here for feeding the plant. The light possibly bounces repeatedly off the insides of leaf walls and enhances the effect.

Thick leaf skins on other parts around the window can then be impenetrable like walls, protective of plant tissue against harsh sunlight in the arid conditions where these plants usually grow (Louis Jordaan, personal communication; Smith, et al, 2017; Vlok and Schutte-Vlok, 2015; iNaturalist; Wikipedia; https://thesucculenteclectic.com).