IAWA Journal, Vol. 15 0), 1994: 51~63 STEM DEVELOPMENT, MEDULLARY BUNDLES, AND WOOD ANATOMY OF CROTON GLANDULOSUS VAR. SEPTENTRIONALIS (EUPHORBIACEAE) by Sheila M. Hayden and W. John Hayden Department of Biology, University of Richmond, Richmond, Virginia 23173, U. S. A. Summary Anatomy and development of vascular tis­ onomically complex assemblage of some 21 sues in the annual stems of Croton glanduZo­ described varieties found from eastern North sus var. septentrionaZis are described. In pri­ America, the West Indies, and South America mary stages of growth the stem possesses a to southern Brazil and northern Argentina; it eustele of bicollateral bundles; internal phloem has been classified in section GeiseZeria (Web­ is notably more extensive than the external. ster 1993). The northernmost element of this In addition to a vascular cambium and sec­ species complex is Croton glanduZosus var. ondary xylem that form in the usual fashion, septentrionaZis Muell. Arg. (Fig. 1), a vigor­ additional cambia add cells to the internal ous weedy annual herb found from Mary­ phloem portion of the bicollateral bundles, land, to Iowa, Florida, and Texas. The plants forming well-marked medullary bundles at grow one to two feet tall and branch freely. the perimeter of the pith. At first, the peri­ Stems of most species of Croton are note­ medullary cambial strands produce only in­ worthy for the presence of internal phloem ternal secondary phloem; later, internal sec­ (Pax 1884; Froembling 1896; Gaucher 1902; ondary xylem is also formed in some stems. Leandri 1939). In addition to these plants, When internal secondary xylem is present, Metcalfe and Chalk (1983) list nine more the medullary bundles have an inverted orien­ genera of Euphorbiaceae said to possess this tation, i.e., phloem innermost (towards centre feature and earlier literature indicates that of pith) and xylem outermost (near proto­ internal phloem or bundles of phloem-like xylem). Cells of the medullary bundles in­ cells are widespread in uniovu1ate members of clude sieve tube elements, vessel ekments, the family (e.·g, Pax 1884; Froembling 1896; and fibres. Normal (external) secondary phlo­ Gaucher 1902, Pax & Hoffmann 1931). The em is weakly developed. Normal secondary most comprehensive descriptions of stem xylem contains short vessel elements with anatomy for Croton and related genera are simple perforation plates and alternate inter­ found in Froembling's (1896) survey (sum­ vascular pits, libriform fibres, narrow hetero­ marised in Solereder 1908 and Metcalfe & cellular rays, and lacks axial parenchyma. Chalk 1950, 1983), but there appears to have Key words: Croton, Euphorbiaceae, internal been no previous study of stem development phloem, medullary bundles, stem devel­ in this genus. Preliminary observations of opment, wood anatomy stems of Croton glandulosus var. septentrio­ naZis confirmed the presence of medullary Introduction bundles containing phloem, but also revealed, Croton L. is a large genus of Euphorbia­ for the first time, some with secondary xylem ceae comprising nearly 1000 species of trees, as well. Thus, the present study was under­ shrubs, and herbs found throughout the taken to provide a detailed first report of stem warm regions of the globe and achieving ontogeny of this species, with an emphasis great diversity in the neotropics (Webster on its vascular tissues, especially the unusual 1992, 1993). Croton glanduZosus L. is a tax- structures encountered in the pith region. Downloaded from Brill.com10/07/2021 03:30:22AM via free access 52 IAWA Journal, Vol. 15 0),1994 Materials and Methods Specimens were collected from the au­ thors' residence in the suburbs of Richmond, Virginia. A series of spring-collected seedling stems provided developmental stages from shortly after expansion of the cotyledons through primary growth, and early stages of secondary growth. Summer and autumn-col­ lected plants yielded stout pencil-thick stems with late stages of secondary growth. Col­ lection data and voucher specimens are as follows: U.S.A., Virginia, Chesterfield Co.: Hayden 2082 (9 July 1988) (URV); Hayden 2634 (26 May 1990) (URV); Hayden 2892 (23 Sept. 1990) (URV). All tissues were fixed in FAA. A total of 15 seedling and herbaceous stems were de­ hydrated in tertiary butanol, embedded in paraffin, and sectioned at 10 11m on a rotary microtome. Paraffin sections were stained in aniline blue and counter-stained with safranin. Six larger stems were sectioned at 30 11m on a sliding microtome and stained in various com­ binations of hematoxylin, aniline blue and safranin. Secondary xylem from the basc of two of the larger stems was macerated by means of Jeffrey's fluid (equal volumes of 10 percent nitric acid and 10 percent chromic acid). Numerical values reported in the wood description are derived from these two sets of macerations as well as sections prepared from the same stems; these values are based on 50 measurements per specimen of vessel element length, diameter, and fibre length, 30 mea­ surements per specimen of end wall angle, and 10 measurements or counts per specimen for all other features. SEM observations were made from sections cut on a sliding micro­ tome at 50 11m, dehydrated in ethanol, critical point dried with liquid C02, and sputter coated with a gold/palladium mixture. The epicotyl of Croton glandulosus var. septentrionalis ascends through several nodes, eventually terminating in a flower cluster sub­ tended by a verticil-like group of smaller flower-bearing stems (Fig. 1). In order to con­ trol for potential ontogenetic variation, i.e., variation with position along the stem, our anatomical observations were routinely fo­ Fig. 1. Croton glandulosus var. septentrio­ cused on the basal portion of the epicotyl, nalis. habit, shortly after onset of flowering. specifically, the most proximal (i.e., first) Scale bar = 2 cm. node and internode above the cotyledons; un- Downloaded from Brill.com10/07/2021 03:30:22AM via free access Hayden & Hayden - Stem anatomy of Croton 53 less stated otherwise all descriptions pertain Nodes conform to the three-trace trilacu­ to this region. Two of the six mature stems nar configuration (Fig. 5). Each trace remains were sectioned at multiple (four or five) inter­ bicollateral as it diverges from the stele and vals spaced evenly between the cotyledons enters the petiole (Fig. 4). Longitudinal sec­ and the first distal cluster of branches to pro­ tions of young stems reveal the presence of vide information on potential developmental sieve tube elements in both internal and ex­ (i.e., positional) variation. ternal phloem. Sieve tube elements are identi­ fiable by virtue of their simple sieve plates stained darkly with aniline blue and conspicu­ Results ous mucous slime plugs (Fig. 11). To the cx­ Primary growth tent detectable in our light microscope prepara­ Primary growth in the stem of Croton glan­ tions, sieve cells of the internal and external dulosus var. septentrionalis produces a eustele primary phloem are structurally similar. Sieve composed of bicollateral bundles (Figs. 2- pores were not observed, probably a conse­ 5). Epidermis is uniseriate and bears few sto­ quence of their presumed small size and the mates but has numerous stellate hairs (Fig. 1, failure to control fixation for optimal preserva­ inset) with multiseriate stalks. Cortex consists tion of fine details in phloem. No lateral sieve of collenchyma and parenchyma cells; in older areas were observed on sieve tube elements. stems, groups of thick-walled laticifers bear­ We assume that small cells among the sieve ing clear latex are prominent in the cortex (Fig. tubc elements of both internal and external 12). Each vascular bundle consists of three phloem are companion cells, but the prepara­ groups of cells, external primary phloem, pri­ tions made do not permit positive confirma­ mary xylem, and internal primary phloem tion of this assumption. (Fig. 3). In the mature primary body, internal phloem is generally more extensive than ex­ Secondary growth ternal phloem (Fig. 3). Internal and external The origin of the vascular cambium locat­ phloem are similar in their total lack of scle­ ed between the last-formed cells of metaxylem renchyma, but external primary phloem has and the external phloem proceeds in a com­ scattered tannin-bearing idioblasts that are pletely normal fashion; the same can be said absent in the internal phloem (Figs. 2, 4, 5). of the accumulation of secondary xylem (Figs. Primary xylem development is exarch and 13, 15). The amount of externally produced consists of single or double files of lignified secondary phloem is sparse and, like the pri­ conducting elements separated by unlignified mary phloem, it lacks sclerenchymatous ele­ xylem parenchyma. Protoxylem walls of ments (Fig. 12), but otherwise its mode of tracheary elements are helically thickened development seems normal. However, an a­ (Fig. 11); scalariformly thickened or pitted typical secondary ontogeny occurs between elements are frequent in the metaxylem. In­ the protoxylem and internal phloem. The com­ ternal phloem excluded, the pith consists bined activity of normal and atypical cambia solely of parenchyma cells (Figs. 2-5). Drus­ transforms the vascular system of the plant es occur scattered throughout the cortex and from a eustele of bicollateral bundles to a pith (Figs. 3-5). Through maturation of the vascular cylinder of normal secondary xylem primary body, pith cells are generally devoid and phloem with the addition of anomalous of starch (Fig. 3). Pith cells from stems with medullary bundles. The anomalous medullary secondary growth, however, are densely bundles consist of the original cells of inter­ packed with starch grains (Figs. 8, 10). nal primary phloem plus additional secondary Developmentally, internal phloem is first vascular tissues. In order to distinguish be­ detectable as a small clump of densely cyto­ tween normal secondary growth and events plasmic cells that differentiate shortly after that occur in the vicinity of the internal phloem, the first elements of protoxylem and external the latter will be referred to as perimedullary phloem.