The arrangement of microtubules in leaves of monocotyledonous and dicotyledonous plants

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The first leaf of Avena sativa L., a monocotyledonous plant, grows in a region that lies within 10 mm of the base of the leaf. Cells in that region elongate longitudinally but hardly expand laterally. The orientation of cortical microtubules in the elongating region is transverse in both epidermal and parenchymal cells. The same features of the arrangement of microtubules are also observed in the leaves of Zea mays. Cellulose microfibrils in the cell wall are coaligned with microtubules, lying approximately transverse to the axis of elongation, as if they function as hoops to facilitate the longitudinal elongation of the cell. The cells of growing leaves of Pisum sativum L., a dicotyledonous plant, expand superficially in every direction at every point on the leaf. Cortical microtubules lining the outer walls of epidermal cells are arranged randomly or in parallel. The parallel microtubules are oriented in various directions. In the outer walls of epidermal cells of growing leaves, areas with different predominant orientations of microfibrils are found within a single cell, consistent with the arrangement of microtubules. These results indicate that the orientation of cortical microtubules is correlated with the orientation of microfibrils and the direction of growth in growing leaves of both monocotyledons and dicotyledons, suggesting the involvement of cortical microtubules in control of the direction of growth in leaves.