Leaf Anatomy and Ontogeny in Argyroxiphilm and Wilkesia (Compositae)

Reprinted from the AMERICAN JOURNAL OF BOTANY, Vol. 44, No. 8, 696-705, October, 1957 Printed in U. S. A. LEAF ANATOMY AND ONTOGENY IN ARGYROXIPHILM AND WILKESIA (COMPOSITAE) Sherwin Carlquist LEAF ANATOMY AND ONTOGENY IN ARGYROXIPHIUM AND WILKESIA (COMPOSITAE)' Sherwin Carlquist THE FOUR SPECIES OF the genus Argyroxiphium occurs in moderately moist habitats on the slopes of (commonly known as the "silverswords ") and the Haleakala, Maui. Wilkesia gymnoxiphium Gray is related monotypic genus Wilkesia are narrowly-re also monocarpic, although its habit is arrestingly stricted Hawaiian endemics. They have been classi different from that of any other Compositae. Its fied with the tarweeds (Heliantheae, subtribe Ma- thin, lanceolate leaves are borne in verticils, with diinae) by Hoffmann (1890), although Keck the leaf bases united into a common tubular sheath (1936) has expressed a dissenting opinion. As in at each node; the stems are long and pole-like. few other genera, remarkable specializations of Wilkesia grows in relatively dry forest areas on habit and habitat make them intriguing objects for Kauai. For photographs and drawings showing the anatomical study. Moreover, anatomical informa habit and gross morphology of these genera, the tion should aid in clarifying their systematic rela reader is referred to Keck (1936), Degener (1946) tionships, as the writer hopes to demonstrate in and Carlquist (1955). ensuing researches. MATERIALS AND METHODS.—Portions of field-col Argyroxiphium sandwichense DC., well known as lected specimens of all species (except A. virescens, the "silversword" because of the shining felty in- which was available as herbarium material) were dument on its leaves, occurs in dry volcanic craters preserved in Carnoy's fluid. This fixative, although on the islands of Maui and Hawaii. It is mono- inadvisable for general work, was excellent here, carpic and has the habit and size of an acaulescent because its anhydrous nature permitted the preser Yucca. The closely related A. caliginii Forbes, vation of intercellular deposits of pectic com from an extremely moist mountain top on West pounds. Supplementary material from a cultivated Maui, is a smaller, low branched rosette plant, not plant of A. sandwichense (courtesy of Dr. H. L. monocarpic. Also in the same location is A. graya- Mason) and Wilkesia (cultivated by the University num (Hillebr.) Degener, which is somewhat shrub of California Botanical Garden from the writer's bier; its leaves, which are flat and wide, lack the seed collections) was preserved in formalin-pro- prominent covering of hairs characteristic of the pionic-alcohol (Johansen, 1940). Leaves of A. two preceding species. Similar in leaf indument to virescens (Forbes 1251M, UC) were treated with A. grayanum is A. virescens Hillebrand, sometimes 2.5 per cent aqueous sodium hydroxide to restore known as the "greensword." Argyroxiphium vi their shape and to remove discoloration. Johansen's rescens, like A. sandwicliense, is monocarpic and tertiary butyl alcohol series, beginning at 85 per has thick, almost quadrangular leaves (fig. 16). It cent alcohol, was used for dehydration, and ma terial was infiltrated and embedded in paraffin ac 1 Received for publication May 15, 1957. cording to the usual techniques. Paraffin ribbons The writer is indebted to Dr. Adriance S. Foster for were mounted with minimal exposure to aqueous helpful suggestions and for critically reading the manu script. Dr. Harold St. John was of assistance during the solutions, and an entirely anhydrous staining series writer's field work in the Hawaiian Islands in 1953. was used. Safranin and fast green dissolved in October, 1957] CARLQUIST—ARGYROXIPHIUM AND WILKES1A (,<><> absolute ethyl alcohol were employed. For identi layer of both surfaces. Such divisions in the adaxial fication of pectic compounds, as well as for a surface are relatively few, and only three layers of permanent stain on sections. Ruthenium red dis palisade parenchyma (fig. 7) arise from this hypo solved in 70 per cent ethyl alcohol was used. The dermal layer. On the abaxial surface, however, pectic nature of intercellular accumulations was periciinal divisions begin early (fig. 3) and are also established by their solubility in warm 0.1 quite numerous, producing files of cells. Active per cent ammonium oxalate; water alone, however, divisions occur to either side of the midvein, al can gradually leach out these compounds. On this though not beneath it (fig. 4) ; this is associated account, and because of the dense trichomes and with the fact that only two pectic channels are fibers in some species, leaves of Argyroxiphium present in the mature leaf. The actively dividing proved exceptionally difficult microtechnical ob cells on the abaxial surface are collectively termed jects. the "abaxial meristem" here. Its products can be Herbarium specimens documenting the writer's delimited by the fact that adjacent to the inner collections have been deposited in the University of surface of cells derived from the abaxial meristem California Herbarium, Berkeley. are dark-staining intercellular pectic deposits (fig. ARGYROXIPHIUM: LEAF ANATOMY.—As shown in 4). As may be seen in longitudinal section (fig. 5), the photograph (fig. 6) or diagram (fig. 16) of the abaxial meristem is only weakly developed to leaf transections of Argyroxiphium, two anomalous ward the base of the primordium (at right). Its features require explanation. First is the presence products include not merely palisade parenchyma, of loosely-arranged mesophyll parenchyma, the in- but also the procambium of the lower set of bun tercelluar spaces in which are partly (fig. 6) or dles, and the lower portions of the pectic channels completely (fig. 11) filled with pectic compounds. (fig. 2, below white lines). At the same time, other This pectic material, although apparently not pres portions of the primordium within the future ent in juvenile leaves of A. sandwichense, is char adaxial palisade produce two series of procambial acteristic of adult leaves of all of the species, and strands and most of the mesophyll parenchyma. As is present in fresh material in jelly-like form. Such fig. 4 shows, the middle bundles of the leaf develop pectic materials are present in channels of meso- first, in sequence from midvein to margins. Pro- |)h\ II which run the length of the leaf between vein cambium originating the upper set of bundles and sheath extensions. The term "pectic channels" is procambium connecting the middle and upper bun applied here to this type of distribution. dles (fig. 2. above) develop next. Simultaneously with maturation of vascular elements, a pair of The second noteworthy feature is the presence secretory canals, sometimes only one, differentiates and orientation of three sets of bundles in the leaf. near the protoxylem of the larger veins (fig. 4, 7). As indicated in both fig. 6 and fig. 16. there are three sets of bundles, designated here as upper, The mature condition is shown in fig. 6 for A. middle, and lower sets. The lower set of bundles .sandwichense. This photograph illustrates a num is inverted in orientation, whereas upper and ber of features which merit mention: (1) the mid- middle sets are normal. vein is provided with a bundle-sheath extension con nected with both epidermal layers, and bundle These anomalous features are best explained by sheaths are developed on smaller veins. (2) along study of the leaf ontogeny. For this purpose, the lower surface, a lower set of inverted bundles preparations of A. caliginii proved most suitable, is present. (3) between the palisade parenchyma although similar features were observed in A. sand- and the pectic channels there are compact, isodia- wwhense. The shoot apex (fig. 1), which is rather melrie parenchyma cells which lack chloroplasts flat, shows two tunica layers. A short distance be and are similar to those of the vein-sheath exten low the central mother cell zone, cells derived from sion. The vascular bundles and fiber strands are rib meristem mature into lacunate pith parenchyma. embedded in this parenchyma. Although some of This short region of meristematic activity is as these strands of fibers are associated with veins. sociated with the rosette habit of the plant. Obser others appear to lack conducting tissues. Some vations on cell lineages in tips of young leaf pri- bundles may appear to be "isolated" within the mordia indicate that marginal and submarginal pectic channels (fig. 6, upper right). These origi initials are present. The submarginal initials give nate from the middle series, and are like the mid- rise to a hypodermal layer on both surfaces of the vein except that they possess no sheath extensions. leaf; they also produce actively dividing cells which Leaves on some plants of A. caliginii and A. sand- soon mature into the numerous layers of the mas wichense are Hatter, and show a slightly different sive mesophyll. In relatively young primordia. structure. In these, veins of the middle set other periciinal divisions can be seen in the hypodermal Fig. 6-9.—Fig. 6. Argyroxiphium sandwichense, Carlquist H28, portions of transection of mature leaf.—Fig. 7. adaxial face above. X96.—Fig. 7-8. A. caliginii, Carlquist H28, portions of tarnsection of mature leaf.—Fig. 7. Portion of adaxial surface. X180.—Fig. 8. Portion of margin. X245.—Fig. 9. A. sandwichense, Carlquist HI9, cells from longitudinal section of pith. X400. 700 AMERICAN JOURNAL OF BOTANY [Vol. 44 Fig. 10-11.—Fig. 10. Argyroxiphium grayanum, Carlquist H27. transection of leaf, a<laxial face above. X128.- Fiii. 11. A. sandwichense, Carlquist III'1, transection of inflorescence bract, adaxial face above. xl33. than the midvein develop complete bundle-sheath ARGYROXIPHIUM: COMPARISON OF SPECIES.—On extensions, so that several, not just two, pectic account of the wide disparity ia habitat between the channels are present. similar pair of species, A. sandwichense and A. In summary, all but the basal portion of adult caliginii, the writer was interested in noting ana leaves of A.

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