Protoplasma 129, 157-167<1985) ~OTOPI.A~ 9 by Springer-Verlag I985 The Development and Ultrastructure of the Testa and Tracheid Bar in Erythrina lysistemon Hutch. (Leguminosae : Papilionoideae) J. C. MANNINGand J. VAN STADEN * UN/CSIR Research Unit for Plant Growth and Development, Department of Botany, University of Natal, Pietermaritzburg Received May 10, 1985 Accepted July 26, 1985 Summary The development of the testa was studied in Erythrina lysistemon using both light and electron microscopy. Cells of the outer epidermis of the outer integument divide anticlinally and undergo radial elongation to form a palisade layer. The outer tangential walls are thickened at an early stage, and deposition of fluted thickenings on the radial walls occurs at maturity. Palisade cells in the hilar region differentiate from sub-funicular tissue, and at maturity the outer ends of the cells undergo extensive deposition of secondary walls and associated lignification. The light line occurs at the junction between the outer, thickened portions of the cells and the inner, iess thickened portions. An electron-translucent (suberised) cap develops in the outer tangential walls of the palisade cells at a late stage. Micro- tubules and dictyosomes are closely associated with the developing thickenings in palisade and tracheid bar, and the microtubules run parallel to the wall microfibrils. Differentiation of the tracheid bar coincides with final secondary wall deposition and lignification in the hilar palisade. The cells of the tracheid bar are dead at maturity, but are surrounded by sheaths of elongate parenchyma. Keywords: Erythrina; Hilum; Light line; Seed structure; Testa development; Tracheid bar. 1. Introduction The seeds of the Leguminosae are typically imper- meable to water, and scarification is required before imbibition and germination can occur (AITKEN 1939, CORNER 1951, QUINLIVAN 1971). This impermeability barrier is usually thought to lie in the palisade layer, and it has been construed variously as a mechanical * Correspondence and Reprints: UN/CSIR Research Unit for Plant Growth and Development, Department of Botany, University of Natal, Pietermaritzburg 3200, Republic of South Africa. (CORNER 1951) or chemical barrier due to suberisation of the outer walls (AITKEN1939, WATSON1948) or to the presence ofphenolics in these cells (WERKER et al. 1979). In addition, the light line has been implicated in preventing water penetration. CORNER (1951) states that the light line indicates a structural discontinuity in the palisade cells between an outer, highly thickened portion and an inner, less thickened portion. HARRIS(1983), however, correlates its position with an osmiophilic zone in the outer tangential wall. Clearly the development of these various structures is essential in understanding their nature and possible function. The structure of the mature testa is quite well es- tablished in a number of papilionoid legumes (HAMLY 1932, AITKEN 1939, MARTIN and WATT 1944, WATSON1948, CORNER1951). Ontogeny of the testa and associated structures has, however, been largely neglec- ted, although developmental studies using the light microscope have been carried out on seeds of Phaseolus lunatus L. (STERLING 1954) and Pisum sativum L. (REEVE 1946). In addition, the partial ontogeny of the epidermal palisade and hypodermal osteosclereids has been studied in detail in Pisum sativum using both light and electron microscopy (HARRIS 1983, 1984). The mature tracheid bar in seeds of the Papilionoideae has received some attention (LERSTEN 1982), but the ontogeny is little studied, and the few studies have relied on light microscopy (STERLING1954). This paper represents the first detailed ultrastructural