NOTE ON THE STRUCTURE OF THE EPICOTYL IN JUGLANS NIGRA

Abstract

T HE following account of seedling structure in Juglans nigra deals with the anatomy of the epicotyl and its early leaves. The work forms part of a study of the seedlings of the Juglandaceae which has arisen out of a previous survey of the seedling anatomy of the Amentiferae suggested by Dr E. N. Thomas. It has been shown that the seedlings of Juglans species are exceptional on account of the part played in them by plumular traces in relation to root poles. In Juglans nigra the traces of the first two plumular leaves enter the axis as bundles which constitute two poles of the tetrarch root independently of the cotyledon traces. The structure and behaviour of these two plumular traces is exactly similar to that which has been described by so many authors as characteristic of cotyledon traces. A similar connection of root poles with plumular traces has been recorded by Compton for two members of the Leguminosae, Pithecolobium unguis-cati and Caesalpinia sepiaria, and, by the present author in Castanea saliva, but apart from these instances the feature seems to be rare or unnoted. It has been found, as described in detail below, that in Juglans nigra doubleness of leaf traces is not confined to the first two plumular leaves, nor is it necessarily concerned in the formation of root poles. A bundle, usually showing isolated centrally placed protoxylem elements, is present in the nodal region of at least six successive plumular leaves. The number of leaves varies slightly from one seedling to another, and recognition of the double bundle depends upon the examination of sufficiently early stages in development. The earliest xylem elements of the lower leaves may be already disorganising before the plumule has emerged from the seed. Double plumular traces unconnected with root poles have been described by Thomas for the first pair of plumular leaves in Cheiranthus maritimus and in Draba Aizoon and by Holden and Bexon as occurring similarly in certain seedlings of Cheiranthus cheiri. Juglans nigra differs markedly from the above-mentioned species in respect of the greater number of leaves which possess the feature in question and the extent to which it persists.