THE RELATIONSHIP BETWEEN THE PRIMARY THICKENING MERISTEM AND THE SECONDARY THICKENING MERISTEM IN YUCCA WHIPPLEI TORR. I. HISTOLOGY OF THE MATURE VEGETATIVE STEM

Anatomical observations were made on 1-, 2-, and 3-yr-old plants of Yucca whipplei Torr. ssp. percursa Haines grown from seed collected from a single parent in Refugio Canyon, Santa Barbara, California. The primary body of the vegetative stem consists of cortex and central cylinder with a central pith. Parenchyma cells in the ground tissue are arranged in anticlinal cell files continuous from beneath the leaf bases, through the cortex and central cylinder to the pith. Individual vascular bundles in the primary body have a collateral arrangement of xylem and phloem. The parenchyma cells of the ground tissue of the secondary body are also arranged in files continuous with those of the primary parenchyma. Secondary vascular bundles have an amphivasal arrangement and an undulating path with frequent anastomoses. Primary and secondary vascular bundles are longitudinally continuous. The primary thickening meristem (PTM) is longitudinally continuous with the secondary thickening meristem (STM). Axillary buds initiated during primary growth were observed in the leaf axils. The STM becomes more active prior to and during root initiation. Layers of secondary vascular bundles are associated with root formation. THE OCCURRENCE of the primary thickening meristem (PTM) is widespread among monocotyledons with a rosette habit (i.e., a wide stem axis with very short internodes) (DeMason, 1983). The PTM has been recognized as a zone of tangentially flattened cells in the upper meristematic portion or crown of the stem. Its presence is correlated with a number of histological features. The two major features are: 1) the apical meristem and youngest leaf primordia are sunken below surrounding stem tissue; and 2) distinctive anticlinal files of cells arepresentinthestem(DeMason, 1979, 1983). Among monocotyledons, the secondary thickening meristem (STM) is not common. The STM occurs in a number of seemingly unrelated families including Liliaceae (Aloe, Aphyllanthesv Veratrum), Xanthorrhoeaceae (Lomandra, Xanthorrhoea), and Iridaceae (Art Received for publication 25 October 1982; revision accepted 14 February 1983. This study represents a portion of a thesis for the M.A. degree in the Department of Botany, University of California, Riverside. We wish to thank Dr. Donald R. Kaplan, Dr. Norman C. Ellstrand, and Dr. Toshio Murashige for critically reading the manuscript. Funding of this study came from NSF Grant PCM 8021422 to Dr. Darleen A. DeMason, principal investigator. Growth chambers were purchased from PHS/BRSc #2 507RR 07010-15. istea, Klattia, Witsenia); but is especially characteristic of the Agavaceae, occurring in all genera of the family (Agave, Beaucarnea, Cordyline, Dasylirion, Dracaena, Furcraea, and Yucca) (Tomlinson and Zimmermann, 1969). The STM is considered to be secondary in nature and to produce secondary parenchyma (secondary conjunctive tissue) bidirectionally (Cheadle, 1937). Entire secondary vascular bundles are thought to diffierentiate from the leafbases within the secondary conjunctive tissue internal to the STM (Zimmermann and Tomlinson, 1970). The histological characteristics of the primary and secondary tissues, and especially the developmental relationship between the primary and secondary vascular bundles in these monocotyledons with secondary growth, are not well understood. The relationship between the STM and PTM is uncertain. Some workers claim that the PTM and the STM are longitudinally continuous (Hausmann, 1908; Helm, 1936; Chouard, 1936; Eckardt,1941). Recently, however, others have reported that the PTM and the STM are discontinuous (Stevenson, 1980; Stevenson and Fisher, 1980) or are initially discontinuous in the seedling, becoming continuous only in later development (Lu and Chiang, 1976; Stevenson, 1980). This paper is the first in a series which in-