AIZOACEAE 207 



Plate 52. 



A. Mesembrianthemum rhopalophyllum Schlechter & Diels A plant with window 

 leaves, in its natural locality in the sandy desert of the southern Namib near Luderitzbucht. 



B. Mesembrianthemum calcareum Marl., among fragments of lime tufa in its natural 

 locality near Kimberley. 



Mesembrianthemum rhopalophyllum. Our little landscape represents this 

 remarkable plant in its natural habitat in the sandy deserts of the southern 

 Namib near Pomona Island. The specimen in front is shown in natural size, 

 one of the leaves being in vertical section. The leaves are club-shaped and 

 consist, like those of other species, of a colourless epidermis, a thin layer of 

 green tissue and a central mass of colourless water-storing cells, which are 

 tightly packed against each other, almost without any intercellular spaces 

 between them. The apex of the leaf possesses, however, no green tissue, 

 but is colourless or greyish, and its epidermis adjoins the water-storing tissue. 



The plant grows embedded in the sand, nothing but the flat, slightly 

 convex apex of each leaf being visible, and even that is often covered with 

 more or less sand according to locality. While the leaf itself is fresh green 

 with a rather delicate skin, the exposed part is protected by a thick epidermis 

 and cuticula, and possesses comparatively few stomata. It is through this 

 portion, which has the function of a window, that the leaf receives its light, 

 being thus illuminated from within. There are 5 to 10 or even more 

 leaves to each plant, but nothing appears at the surface except these windows ; 

 they peep out of the sand like the eyes of the sand-lizard or the sand-viper, 

 which often hide themselves there in a similar way. 



When the flowering season arrives (August) one may see the delicate, 

 white, glossy blooms even where no windows are visible ; but they are there 

 all the same, merely hidden by a little sand. 



The subterranean mode of life which the plant has adopted affords it 

 considerable protection against herbivorous animals, especially at night time, 

 since it would be difficult for them to detect these little spots in a dim light. 

 To the non-botanical reader it may be pointed out, that the window is an 

 essential feature in this adaptation, for if the apex were provided with green 

 tissue, this would absorb the light falling on it and the buried portion of the 

 leaf would be colourless, or if green before it was buried, would become 

 colourless like the underground shoot of a potato, and consequently unable to 

 assimilate the food materials provided by air and soil. 



There are a few other species of Mesembrianthemum with window leaves, 

 e.g. M. Hookeri (Plate 51), M. pseudotruncatellum and M. opticum\ a few species 

 of Haworthia, e.g. H. truncata, H. tesselata (Vol. iv, Plate 22), and one of 

 Bulbine, viz. B. mesembrianthemoides (Vol. iv, Plate 27), but as far as we have 

 been able to ascertain no such plants are known from other countries. 



M. 27 



