Histological Studies on the Root of Melilotus alba

Abstract

In the ontogeny of the root of M. alba the primary tissues develop from a general meristem. Protophloem is the first stelar tissue differentiated. In seedlings lateral roots arise in the pericycle and possess a temporary root cap formed by the endodermis. In older roots and in the hypocotyl lateral roots develop in the periderm and primary rays. A cork cambium, arising in the endodermis, and a vascular cambium form secondary tissues. Tertiary tissue may develop at the center of the tap root. Proliferation of parenchyma late in the first season is accompanied by rapid increase in the diam. of the tap root and hypocotyl, accumulation of a large reserve of starch, and withdrawal of the hypocotyl underground. A pith-like mass, not homologous with the pith of the upper part of the hypocotyl and stem, develops at the center of the stele. Secondary growth continues in the 2d yr. through the period of flowering, but more slowly. Degeneration occurs after seeds are formed, involving late disappearance of the starch reserve, crushing of phloem, isolation of the periderm by a layer of dead cells, thickening of the walls of parenchyma cells and final disintegration of tissues by saprophytic fungi.