The Lithops lobes derived from ordinary plant leaves: The top side transmits light and is hard coated for protection, the bottom side has microscopic apertures (stomata) for gas exchange. This is the same with Lithops, besides the "leaves" have got a huge water storage layer within. It is fully transparent, and photosynthesis is performed by the green cells near the sidewalls, which are the bottom side of the "leaf". Lithops are CAM-Plants, that means that they perform photosynthesis and gas exchange differently scheduled, to keep the stomata closed at daytime to reduce transpiration. The Lithops top side is called window due to its explicit transparency. Perfect: A red tinted window! On the one hand it transmits red light for photosynthesis and on the other it prevents the plant from looking green, so plant-eating animals won't find it.