Root segments of Convolvulus arvensis, the field bindweed, were examined with the electron microscope to make possible a description of the fine structural correlates of lateral root protrusion through cortical parenchyma. Particular attention was directed to the outermost primordium cells, derived by meristematic activity from the endodermis, and to the contiguous cortical parenchyma cells. By following the fate of the Casparian strip through numerous divisions of the endodermal cell, information has been obtained relating to the minimum contribution of the endodermis to the root primordium structure. Cortical parenchyma cells during lateral root growth are specifically degraded so that only the cell wall remains. A layer of cell wall material, representing numerous cortical parenchyma cells, accrues at the tip of the advancing root primordium. It is suggested that the intensive coated-vesicle activity along the plasmalemma of the outermost primordium cell and the appearance of the vesicle contents in the outer wall of this cell are indicative of the secretion of hydrolases which move through the wall and attack the adjacent cortical parenchyma cells.

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