LIGNOTUBER ONTOGENY IN THE CORK-OAK (QUERCUS-SUBER, FAGACEAE) .2. GERMINATION AND YOUNG SEEDLING

Lignotuber origin was studied in cork-oak seedlings at different stages from germination to seedling establishment. By the time germination is completed, the seedling consists of an elongated embryonic axis between the developing first shoot and first root. The cotyledonary insertion divides the axis in two portions: an apical portion, visible between the cotyledonary petioles; and an enlarged lower portion in which the cotyledonary tissues are fused with the axis. Several histological changes take place in the fused portion: initial vascular differentiation occurs; parenchymatous cells contain large amounts of starch; and the vascular cambium and periderm are formed early. In the cotyledonary stage seedlings, axillary buds of the cotyledonary node develop. Buds in the unfused upper portion remain single and form a short stalk and laminar scales. Buds in the fused portion, hidden by the cotyledonary tissues, form hypertrophied scales and multiply to form bud clusters. As development proceeds, the root-shoot transition region becomes woody, bearing both individual buds as well as bud clusters and concentrating food reserves. All of the transition region in the cork-oak is considered equivalent to a lignotuber.