Abstract
Histological and anatomical observations were made on plants ranging from embryos to 1‐yr‐old plants of Yucca whipplei Torr. ssp. percursa Haines. Our objective was to determine the time and position of origin, ontogenetic relationship, and function of the primary thickening meristem (PTM) and the secondary thickening meristem (STM) during development of the vegetative axis. The PTM arises in the stem periphery during germination. It is longitudinally continuous from beneath the youngest leaf primordia to the primary root, and functions in the production of primary vascular bundles and ground tissue. The STM arises ontogenetically from the PTM in the base of 2‐ to 3‐month seedlings and produces secondary vascular bundles and ground tissue. The PTM and STM, primary and secondary vascular bundles, and files of primary and secondary ground tissue are continuous at all developmental stages studied. Development of the primary body is histologically and quantitatively similar to development in monocotyledons which possess only a PTM; and secondary growth is interpreted as a developmental continuation of the process of primary thickening.
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