SCLEREID DEVELOPMENT IN CAMELLIA PETIOLES

Sclereid initials are distinguishable from neighboring cells by their large centralized nuclei. During the younger stages of development, they are densely cytoplasmic and have an abundance of cellular organelles-mitochondria, endoplasmic reticulum, Golgi bodies, and ribosomes. In maturing sclereids, plastids occur infrequently and have few inner membranes, and the nuclei display unique undulations. Secondary wall synthesis occurs during maturation or with the cessation of apical intrusive growth of branches. The secondary wall differs from the primary wall by its hyaline region, homogeneous and electron-translucent appearance and presence of numerous pit canals. SCLEREIDS are sclerenchyma cells which vary in form, degree of elongation, and secondary wall thickness and are found in various plant organs. Trichosclereids have been investigated in the air root of Monstera deliciosa (Bloch, 1946), and single or grouped sclereids are common in fruits where they are interspersed with the parenchyma of the mesocarp, constitute the stony endocarp, or strengthen the seed coat. Sclereids are also components of the cortex, pith, xylem, and phloem of gymnosperm and dicotyledon stems. Many species of plants develop foliar sclereids, and their occurrence is documented in the standard reference works on plant anatomy (Cutter, 1978; Esau, 1965, 1977; Metcalfe and Chalk, 1950). Foster (1944) used light microscopy to study the foliar sclereids of Camellia japonica. He found that sclereid initials are distinguished from other cells by larger nuclear size and are detectable in leaves 5-6 cm in length. Foard (1959) carried out an additional investigation on cleared leaf preparations of C. japonica and established that sclereid initials occurred predominately along the leaf margins, both natural and surgically incised. He concluded that relative position and osmotic pressure were probable factors affecting sclereid differentiation. Few ultrastructural investigations of sclereids have been made. Parameswaren (1975) studied the wall structure of sclereids in some tree barks, and Ledbetter and Porter (1970) published a single electron micrograph of a pear brachysclereid. Electron microscopy of fibers, which intergrade with sclereids (Esau, 1965), has concentrated on the development 1 Received for publication 12 November 1980; revision accepted 16 January 1981. The authors gratefully acknowledge the support granted by the University of Arkansas Research Reserve for this project. of secondary xylem fibers in woody plants. Ultrastructural investigations on Acer rubrum (Cronshaw, 1965), Coleus (Pizzolato and Heimsch, 1975), and Lolium temulentum (Lawton and Harris, 1979) show that fibers develop from elongate cells with abundant cytoplasm containing many ribosomes, few large mitochondria, proplastids, Golgi bodies, and either several small or one large central vacuole depending on the stage of differentiation. This investigation describes the ultrastructure of the cytoplasm and cell walls of developing sclereids from petioles of C. japonica. Materials in different stages of maturation were examined to elucidate the differentiation process which separates sclereids from surrounding cell types. MATERIALS AND METHODS-First year leaves in various stages of development were collected from greenhouse-grown plants of Camellia japonica. Petioles were cut into 0.5cm segments and processed according to the following procedures. Paraffin techniques (light microscopy) Petioles were fixed in FAA and embedded in Paraplast by standard methods (Johansen, 1940). Sections were cut at 10-12,4m with a Sorvall JB-4 microtome and stained with either safranin-fast green (Sass, 1964) or toluidine blue 0 (Sakai, 1973). Epoxy techniques (light and electron microscopy)-Petioles were fixed in 4% paraformaldehyde-glutaraldehyde in 0.2 M cacodylate buffer at pH 7.2 for 72 hr under vacuum. After a buffer rinse, the tissues were postfixed in 2% OSO4 for 12 hr and bulk-strained in 0.2% aqueous uranyl acetate overnight. The materials were dehydrated in a graded ethanol-propylene oxide series, infiltrated under vacuum, and embedded in an epoxy resin (Spurr, 1969).