FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 327 



A further reduction of the evaporating surface is brought about by the 

 development of thickened or fleshy leaves. In order to render the points under 

 consideration as clear as possible, it is perhaps well to insert here the following 

 observations. By altering the form of a sheet of lead 8 cms. square and 1 mm. 

 thick into a solid cylinder, the diameter of this cylinder is seen to be only 1 cm., and 

 the whole surface of the cylinder is only one-fifth of that of the previous flat sheet. 

 The application of these figures to the tissue of a leaf demonstrates how much 

 smaller is the transpiring surface of a thick cylindrical leaf than of a thin flattened 

 one. Such thickened leaves, which approach more or less to the cylindrical shape, 

 are to be found regularly where transpiration has to be reduced for a considerable 

 time — as, for example, in the mountainous districts of Central and Southern Europe, 

 in the genus Sedum, growing on sandy soil which easily dries up, and on stone walls 

 and battlements (Sedum, album, rejlexum, dasyphyllum, atratum, Boloniense, 

 Hispanicum, &c). They also occur in a striking manner in many tropical orchids 

 which grow on rocks, or epiphytically on the bark of trees in the East Indies, 

 Mexico, and Brazil, exposed for more than six months to great aridity (Brasavola 

 cordata and tuberculata, Dendrobiwm junceum, Leptotes bicolor, Oncidium 

 Cavendishianum and longifolium, Sarcanthus rostratus, Vanda teres, and many 

 others) ; but especially are they found in aloes and stapelias and species of 

 Cotyledon, Crassula, and Mesembryanthemum, whose habitat is in the dryest 

 districts of the Cape. Several Uinbelliferse, Conipositse, and Portulacese {Inula 

 crithmoides, Crithmum maritimum, Talinum fruticosum) growing on stony places 

 of the sea-shore in the burning sun, and many salsolas of the deserts and salt 

 steppes, as well as finally some Proteacese, which for two-thirds of the year are 

 exposed to the droughts of Australia — all are characterized by their development of 

 fleshy leaves. 



Just as thick-leaved plants have acquired their succulence by a modification 

 of their foliage, similarly, in the so-called cactiform plants, it is the stems which 

 become thick and fleshy, and take on the functions of leaves. Here the green 

 tissue is situated in the cortex of the stem, the epidermis covering it contains 

 stomata just like the epidermis of foliage-leaves, and the green cortex transpires, 

 and functions on the whole exactly as the green leaves do. When the stems 

 of the cactiform plants are richly branched and the branches are short, they 

 sometimes much resemble thick -leaved plants. Frequently also the separate 

 portions of the stem and branches take the form of fleshy leaf-like discs, as in the 

 genus of the Prickly-pear (Opuntia), and such stem-structures are usually mis- 

 taken by the uninitiated for thick leaves. Gardeners, as a rule, group the thick- 

 leaved and cactiform plants together under the single term "succulent plants". 

 To the cactiform plants belong the opuntias and cacti, species of Gereus, Echino- 

 cactus, Melocactus, and MammiUaria, which are distributed from Chili and South 

 Brazil over Peru, Columbia, the Antilles, and Guatemala. These are, however, 

 especially developed on the high plains of Mexico in astonishing variety of form. 

 To the cactifoi'm plants belong also the leafless candelabra-like tree-shaped spurges 



