LEAF ANATOMY OF WELWITSCHIA. II. A STUDY OF MATURE LEAVES

Abstract

THE LEAF OF a mature plant of Welwitschia has Lhe same basic pattern, with some modification of cell types, as that of the oldest seedling leaves studied (Rodin, 1958). All leaves have isolateral symmetry, but only leaves of older plants contain numerous sclereids. Hooker (1863) gave the first description of a mature leaf, and other workers have made additional contributions (Pearson, 1929; Sykes, 1910a; Takeda, 1913a). The structure of the leaf will be discussed according to the three primary tissue systems. THE EPIDERMAL SYSTEM.-The epidermal system consists of a single layer of cells on the upper and lower surfaces of the leaf. From the surface epidermal cells appear polygonal with from three to five sides. The lateral walls are thickened, the corners rounded (fig. 8). At the leaf base epidermal cells form neat rows (Takeda 1913a, fig. 4), but cells in more distal parts of the leaf are arranged with less regularity. Three distinct layers are present in the heavily thickened outer wall of epidermal cells as observed by Bleisch (1891), Sykes (1910a), Takeda (1913a), and by this author (fig. 9, 10). First, a thin outer non-granular cutinized layer occurs, in which no laminations can be detected. Apparently it is not a waxy layer external to the cell wall, but consists of particles of cutin laid down within the wall. Secondly, there is a thick, middle granular cutinized layer in which granules have been considered to be calcium oxalate by Solms-Laubach (1871) and Bleisch (1891). The third layer, very thick in mature material, is non-granular, noncutinized, laminated, and innermost. Lignin was not detected by this author in any of the layers and cellulose is probably the chief constituent of epidermal cell walls. The cutin in the two outer layers was detected by the use of Sudan IV, a specific stain for fatty substances. Takeda (1913a) reported on the ontogenetic development of the stomata: the two guard cells and the subsidiary cells originate from a single epidermal cell. In the present research it was observed that only after the guard cells begin to elongate do they assume their characteristic sunken position. Tests with Sudan IV reveal cutin in the outer cell walls of the subsidiary and guard cells, as in other epidermal cells. However, in disagreement with Ta-