ACTIVITY OF MARGINAL AND PLATE MERISTEMS DURING LEAF DEVELOPMENT OF XANTHIUM PENNSYLVANICUM

A B S T R A C T The mitotic and biosynthetic activities of the marginal and plate meristems were studied during the entire course of leaf development of Xanthium pennsylvanicum. In contrast to statements in the literature, marginal meristem activity is long in duration, as assayed by the mitotic counts and H3-thymidine incorporation. This me istem is active 23 days. The plate meristem is active for an additional 3 days after cessation of cell division in the marginal meristem, but the total duration of its mitotic activity is also approximately 23 days. Numerous periclinal cell divisions of the plate meristem form additional cell layers and contribute to the growth of the lamina in thickness. Incorporation of H3-thymidine increased during the course of leaf development. Cells between plastochronic ages 0 and 2.0 incorporated more of the radioisotopic precursor than those of younger leaf primordia. The uptake and incorporation of H3-thymidine into nuclear DNA was more sluggish during the early stages of development than in the more expanded leaves. No DNA synthesis was demonstrated after cessation of cell division in the leaf lamina. Metabolic or endomitotic DNA synthesis after leaf plastochron index (LPI) 3.0 seems improbable. No significant differences in the incorporation of H3-thymidine could be demonstrated between the marginal and plate meristems. This would indicate no distinct biosynthetic differences between the two meristems. The definitions of the marginal and plate meristems of Xanthium leaves were formulated in view of the above findings. Two MERISTEMS are active in the formation of the lamina during the early stages of leaf development. The marginal meristem, represented by a group of meristematic cells located on two flank sides of a young leaf primordium, is active in establishing usually five or six basic layers of the lamina (Esau, 1965). The plate meristem (Schiiepp, 1966), by its numerous anticlinal cell divisions, contributes to the extension of the leaf blade, in which the number of originally established cell layers remains unchanged for a considerable length of time. The flat blade of the leaf is a result of the activity of the plate meristem. Little information is available concerning the temporal aspects of the mitotic and biosynthetic activities of the marginal and plate meristems as related to the various stages of leaf development.