THE RELATIONSHIP BETWEEN THE PRIMARY THICKENING MERISTEM AND THE SECONDARY THICKENING MERISTEM IN YUCCA WHIPPLEI TORR. II. ONTOGENETIC RELATIONSHIP WITHIN THE VEGETATIVE STEM

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 2to 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. This content downloaded from 207.46.13.51 on Sun, 19 Jun 2016 07:41:27 UTC All use subject to http://about.jstor.org/terms [Vol. 70 1 206 AMERICAN JOURNAL OF BOTANY Brebner (1893) claim that the STM diffierentiates in the outer layer of the central cylinder, yet in the same paper they also report that tangential divisions continue in the region they claim is mature primary tissue. Barkley (1924) also reports that the STM arises in the outer layer of the central cylinder, but he does not state the position and characteristics of the PTM. The time and position of origin of the STM in relation to the PTM during seedling growth has been discussed in only a few studies. Hausmann ( 1 908) and Wright ( 1 90 1 ) report that the STM arises in the hypocotyl but they do not describe the development of the PTM. Stevenson (1980) reports that in Beaucarnea recurvata, the STM arises in the cortex of the hypocotyl and, subsequently the PTM diffierentiates in the apex. He states that the "PTM has an origin that is temporally and topographically distinct from the STM," but that the meristems become longitudinally continuous only to lose this continuity in the mature axis. However, the evidence presented in his paper is inconclusive. Stevenson relies solely on position within the stem to identify a meristem as primary or secondary. As we have stated, the meristems are histologically similar and can be distinguished only by the type and arrangement of the vascular bundles produced. In the present study, histological and anatomical observations ofthe meristems and their derivative tissues are correlated in order to define more clearly the relationship between the STM and PTM during development of the vegetative axis of Yucca whipplei. The relationship in the mature axis is described in the first paper of this series. The specific questions addressed here are: 1) What are the time and position of origin of the PTM and STM? 2) What is the ontogenetic relationship ofthe STM to the PTM during development of the axis? 3) How do these meristems function during growth and development from germination to vegetative rosette? MATERIALS AND METHODS-Seed of Yucca whipplei Torr. ssp. percursa Haines was collected and germinated as described earlier (Diggle and DeMason, 1983). Plants were harvested at regular intervals from germination to 12 months. Germination was taken to be the point at which the first true leaf emerged from the sheath of the cotyledon and was visible at the soil surface. Early stages (embryos to 6 wk) were fixed in 2% glutaraldehyde in 50 mM phosphate buffer at pH 7.2, dehydrated in an ethanol series, and embedded in Sorvall or Polysciences JB 4 methacrylate. They were sectioned serially on a Sorvall ultramicrotome and stained with Toluidine Blue (O'Brien and McCully, 1981 ). Two-month to 1 2-month-old plants were fixed in FAA, dehydrated in an ethanol series, embedded in paraplast, sectioned serially on a rotary microtome, and stained with safranin and fast green. Photographs were taken on Kodak Ektapan sheet film through a Zeiss standard microscope and a Nikon Multiphot system. Quantitative measurements were taken from median longitudinal sections of each plant. Sample sizes were 7 (6 wk, 4 and 12 months), 8 (4 and 10 months), 9 (2, 3, 6, and 8 months) or 14 (1 month). Stem height was measured from the base of the vasculature to the center of the apical meristem; stem width was measured at the greatest stem diameter. To determine the developmental events that occur at a given level in the stem, standard stem heights were chosen (2.0, 4.0, and 6.0 mm) to observe: 1) the number of cells/file in the cortex; 2) the diameter of the central cylinder; and 3) the angle of cell files in the central cylinder and cortex. The angle of cell files was measured as the angle from the vertical. As cell files develop a curved path during secondary growth, only the straight primary portion of the files was measured. Analysis of variance was done and found to be significant. Duncan's multiple range test was used to determine significant diffierences between means. Means are plotted on graphs and significant differences are indicated. RESULTS-Stem development-The embryo has a prominent radicle, an apical meristem, and a single ensheathing cotyledon (Fig. 1). During germination, the short hypocotyl elongates, the first true leaf is initiated and expands through the cotyledonary sheath, an additional leaf primordium is initiated, and procambial Fig. 1-6. Median longitudinal sections through embryo and seedling stages; scale = 0.2 mm. 1. Embryo. x40. 2. Seedling at germination. X50. 3. One-wk-old seedling. x45. 4. Two-wk-old seedling. x40. 5. Four-wk-old seedling. * indicates an obliquely oriented vascular bundle associated with the first shoot borne root. X35. 6. Six-wk-old seedling. x28. C, cortex; CC, central cylinder; CT, cotyledon; L, first true leaf; PS, procambial strand, R, radicle, RI, site of root initiation; SA, shoot apex; unlabeled arrows indicate cell files of meristen. This content downloaded from 207.46.13.51 on Sun, 19 Jun 2016 07:41:27 UTC All use subject to http://about.jstor.org/terms 1207 September, 1983] DIGGLE AND DEMASON-YUCCA MERISTEMS II