"While searching for [pebble plants in the Namib Desert], you may find instead what looks, at first sight, to be a fragment of an ancient mosaic pavement, a group of button-sized discs fitting closely together, each a pellucid flinty grey elegantly rimmed with red. Blow hard on them and the sand disperses to reveal that these are the flat tops of a group of rod-like pillars, several inches long. They are, in fact, leaves and this is a window plant…When the plant first germinates, it puts down a short but muscular root. Having got a grip on the surrounding soil, it contracts, pulling down its terminal bud below ground level. The leaves that sprout from it are thick and succulent and absorb moisture as soon as it becomes available. Keeping cool below the surface they are well placed to do that. They are not so well placed to photosynthesise since only their tips are in the light. But those tips are not only flat but translucent. The sunlight falling on them passes through them and then down a series of aligned transparent crystals of oxalic acid running down the center of the pillar until it reaches the grains of chlorophyll that are distributed internally around the sides and the bottom of the leaf."
(Attenborough: Private Life Of Plants 1995:265-267)