The leaf of a typical mesophyte (2.9) has a thin, flat, expanded green area (the blade or lamina) supported on a stalk (petiole) from whose base on the upper side there is a bud. Additionally, there may be two small lateral outgrowths at the base of the petiole, one on either side (the stipules). The arrangement of leaves on a stem (phyllotaxy) is spiral, or alternate, or opposite in pairs, or sometimes whorled — three or more in a ring, as in Peperomia galioides. Gasteria( 13.8) shows the leaves clearly in two series and alternating. Where successive pairs of opposite leaves are set at right angles up the stem, as in Crassula arborescens (2.19), they are said to be decussate. Foreshortening of the rr
spiral leaves into a rosette (Echeveria 2.2) and decussate leaves into a square column (Crassula teres, C. columnaris 2.7). The rosette habit is common in leaf succulents, as it is in alpines and many persistent weeds such as daisies and plantains. Greenovia and some other rosette succulents close up like a bulb when dried out. the dead outer leaves effectively protecting the bud in the centre.
Leaves in succulents vary in size from microscopic scales to those of some agaves, which reach 2m (6Hft) or more in length. 0.5m (20in) in width at the base and a proportionate thickness. With their sharp terminal prickle and saw-like margins they present a formidable defense (2.16). Some species of Anacamp-seros show a curious specialization (12.3) in which the tiny, scale-like green leaves are completely enveloped by the white papery stipules—an effective means of lessening evaporation and reflecting some of the light.
Numerous unrelated succulents display translucent stripes or patches on their foliage. In Pilea each tiny leaf has a transparent underside as if a droplet of water hung from it. Peperomia dolabri-formis has a clear ridge along the top of each vertically compressed leaf, and various haworthias have translucent leaf tips, best seen when the plant is held up against the light (2.6). The most advanced "windows" occur in some Mesembryan-themaceae (Fenestraria 11.21, Frithia 11.22. Conophytum. Lithops). in Haworthia ntaughanii and H. truncata (13.12) and in Bulbine mesembryanth-oides. Here the leaf rosette is naturally buried in the ground with only the clear windowed tips exposed. The chlorophyll-ous tissue is restricted toan area near the leaf surface below ground, and the plant seems to have a crude optical system that permits incident light striking the window to be diffused by crystals of calcium oxalate on to the green area beneath.
Leaves of deciduous succulents— notably the caudiciform types—may look little different from those characteristic of mesophytes. Because the leaf dropsat the onset of drought, measures to limit transpiration are unnecessary. However, most leaves of succulents are strongly xero-morphic: simple in outline, hardly ever finely divided like those of a fern, and often sessile (without a stalk). The stipules, if developed at all, are often modified into protective structures: hairs in Portulaca. spines in some euphorbias. Jatropha (21.15) and Pereskia aculeata (page 178). A sphere has the minimum surface in relation to volume, and thus the ultimate surface reduction is found in plants with spherical leaves (Senecio rowleyanus) or spherical leaf pairs (Conophytum 11.29). In consequence of the fleshiness, veins are rarely perceptible. Sempervivum montanum and species of
Psammophora have the leaves covered in sticky resinous glands that pick up windblown particles of soil and sand. This "mudpack" is not an aid to beauty but makes the plants less conspicuous and doubtless reduces transpiration.
Some succulents have leaves of two distinct types: we say they are dimorphic. In Mitrophyllum (11.11) and allied genera, leaves of the resting period expose less surface for evaporation than those at the height of the growing season. Another type of dimorphism occurs in Gasteria. where juvenile foliage is so unlike the leaves on adult shoots that it is hard to believe both belong to the same species.
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