COMPARATIVE ANATOMY OF THE SEED COATS OF GNETUM AND THEIR PROBABLE EVOLUTION

Abstract

The seed coats of Gnetum gnemon L., G. ula Brongn., G. montanum f. parvifolium (Warb.) Mgf. and G. neglectum Bl. consist of three layers. The outer layer or sarcotesta is mostly parenchyma but contains some sclereids and fibers and a series of simple vascular bundles. The middle sclerotesta forms masses of sclereids in varying shapes and numbers, sometimes extending as a basal plate, and is usually thicker near the micropylar tube. The second layer also contains a series of small vascular bundles that reach the apex. Depending on the species, the middle layer is sometimes nearly free from the outer layer, may be partially fused with it, or completely fused to it at maturity. The innermost layer of the seed coat constitutes the endotesta which is membranous and only rarely contains sclerenchyma. It possesses a dichotomous venation system with varying degrees of anastomosing, depending upon the species. The above species show qualitative and quantitative differences in their sclerenchyma and laticifers. Seed coat anatomy may be useful in the diagnosis of some species. The trends of evolution of seed coat structure within these four species of Gnetum are discussed, and a comparison of tissue layers and vasculature with certain fossil pteridosperms is made, especially in the Trigonocarpales