Leaf Histogenesis in Lactuca Sativa With Emphasis Upon Laticifer Ontogeny

The leaf primordia of Lactuca sativa ‘Meikoningen’ develop from a subapical initial in the second layer of the tunica on the side of a fiat shoot apex. Subsequent growth of the subsurface lamina is initiated by submarginal initials which divide anticlinally to produce an adaxial layer and ***a biseriate abaxiallayer, and periclinally to produce a middle layer from which procambium differentiates. The protoderm is derived from the first tunica layer by continuous anticlinal divisions. The activity of the subapical and submarginal initials is completed when the leaf is 0.3 mm in length and 4.0 mm in width, respectively. Continued growth of the leaf to 130‐150 mm results from intercalary cell division and enlargement. The mature venation is visibly delineated when the leaf is 25‐30 mm in length. Laticifer and phloem cells are initiated by the same mother cells in the ***procambium. The former become non‐septate laticifers by resorption of cross walls. They mature concurrently with the phloem and before the xylem.