IAWA Bulletin n.s., Vol. 4 (2-3), 1983 131 DEVELOPMENTAL CHANGES IN THE WOOD OF BOCCONIA VULCANICA DONN. SMITH by Billy G. Cumbie Division of Biological Sciences, University of Missouri, Columbia, Missouri 65211, U. S. A. Summary Developmental changes in the xylem were xylem differs in a number of respects from studied in a stem of Bocconia vulcanica Donn. the wood in primary woody plants (Carlquist, Smith with a xylem radius of 3.0-4.5 cm. 1974). Growth rings are absent. The vascular cambium One of the families of dicotyledons that is nonstoried with fusiform initials averaging shows the transformation of herbaceous plants 282 J.Lill long. The specialised vessel members into woody plants is the Papaveraceae (Takhta­ are short, with oblique to transverse end walls, jan, 1980). This family consists predominantly simple perforations, and alternate intervascular of herbs, although species of Dendromecon pitting. Vessels are relatively uniform in diame­ tend to be shrubby, while those of Bocconia ter and arrangement throughout the wood. are shrubs, small trees, or woody herbs (Law­ Fibres have moderately thin walls and do not rence, 1951). Bocconia consists of ten species increase in length from the primary xylem to in tropical America (Standley & Steyermark, the cambium. Axial parenchyma is paratracheal, 1946). Halle et al. (1978) list Bocconia as an scanty to vasicentric. Rays are exclusively mul­ example of Corner's model, a monocaulous tiseriate, tall, and heterocellular with a predo­ tree with a single aerial meristem producing an minance of erect and square cells. Sheath cells unbranched axis with lateral inflorescences. occur along the sides. There are no fibres in the However, Standley and Steyermark (1946) de­ secondary phloem and a periderm is not pres­ scribe the inflorescence in Bocconia as being ent. The xylem and bark are similar in many terminal. A plant of Bocconia frutescens L., respects to that formed in some groups of dico­ growing in a greenhouse, is monocaulous, con­ tyledons that are basically herbaceous with sisting of a linear sympodium. After producing evolution toward woodiness. a stem with several internodes, a terminal inflo­ Key words: Secondary xylem, woodiness, rays, rescence forms. A new lateral branch forms paedomorphosis. at a node at the base of the inflorescence, and pushes the inflorescence to one side as it grows Introduction upward. Thus, Bocconia is an example of the One of the major trends of evolution in the Chamberlain model (Halle et aI., 1978). dicotyledons has been the derivation of herba­ This paper describes the developmental ceous species from woody forms. However, this changes occurring in the wood of Bocconia vul­ evolutionary trend is not irreversible. Carlquist canica Donn. Smith, a small tree 3-8 m tall, (1966, 1974), Cronquist (1968), Stebbins from the mesic mountain forests of Guatemala. (1974), and Takhtajan (1980) point out that Some observations are also made of the bark. trends in growth habit have been reversed in The main objective is to compare the structure several unrelated families of dicotyledons that of the xylem in Bocconia with that in second­ are basically herbaceous, including, for exam­ arily woody species of dicotyledons to deter­ ple, the Compositae, Campanulaceae, Cheno­ mine if it is likely that this woody form has podiaceae, and Phytolaccaceae. The evolution evolved from herbaceous ancestors. of woody forms from herbaceous ancestors has been in response to highly specific ecological Materials and Methods conditions, such as adaptation to mesic, rela­ Material of Bocconia vulcanica was collected tively uniform conditions on islands and conti­ by W.E. Harmon, 3592 (University of Missouri nents (Carlquist, 1974). Herbarium, 92676), August 6, 1970, from a Woody species that are derived secondarily mesic mountain forest on the north slopes of from herbaceous forms have limited cambial Volcano Agua, Sacatepequez, Guatemala. From activity and are typically smaller than primary the material, a dried oval stem 8.5 x 10 cm in woody plants. Although they may be similar in diameter was used for this study. The stem had growth form to truly woody plants, they in­ a massive pith, 1.4 cm in diameter, and a xylem clude unusual forms such as rosette trees and radius of 3.0 x 4.5 cm. A relatively thick bark shrubs, and 'woody' herbs. The secondary covered the stem. Downloaded from Brill.com09/28/2021 08:13:39AM via free access 132 IAWA Bulletin n.s., Vol. 4 (2-3),1983 Along the 4.5 cm xylem radius, two rectan­ Usually there are no vessels in these strands. gular blocks 7 mm wide and about 10 mm deep Lateral to these strands, and separating them were cut. One block was divided into five from the large strands containing vessels, are pieces, each with a radial thickness of 9 mm, interfascicular areas or medullary rays 2-3 cells and prepared for sectioning using standard par­ wide (Fig. I). These extend the entire height of affin techniques (Sass, 1958). Several trans­ the sections of about 10 mm, and consist of verse, tangential, and radial sections were cut thin-walled, extremely erect cells. from each piece and stained with hematoxylin Growth rings are absent in the secondary xy­ and safranin. The other block was used to pre­ lem. Vessel members I mm out from the pri­ pare macerations at 5 mm intervals along the mary xylem are markedly shorter than those in xylem radius using Jeffrey's method (Sass, the outer primary xylem (Fig. 13). During the 1958). Similar blocks were cut along the 3 mm production of the first 20 mm of secondary xy­ radius. After dividing one block into six pieces lem, vessel member length shows a slight but (five wood and one bark) and embedding in continuous decrease. Subsequently, vessel mem­ paraffin, several transverse sections were cut ber length remains relatively constant at about off each piece. Each piece was then remounted 290 pm (Fig. 13). The vessel members are spe­ and completely cut into serial tangential sec­ cialised, with oblique to transverse end walls, tions 10 pm thick (about 3000 wood and 720 simple perforations, and rather large, alternate bark sections). The other block along the 3 cm intervascular pitting. During the formation of radius was used to prepare macerations at 5 mm the first mm of wood, vessel diameter increases intervals. Some additional transverse sections to an average of 120 pm, and remains uniform were cut from small pieces of wood embedded throughout secondary growth (Fig. 13). The in glycol methacrylate (JB-4, Polysciences Inc., distribution of vessels is about the same in all Warrington, PA). These sections, 3-5 J.1.m thick, the secondary xylem. Solitary vessels, multiples were stained with toluidine blue. and small clusters occur in about equal num­ Average values for secondary xylem vessel bers. In the inner 5 mm of wood, there are ap­ member length and fibre length are based on proximately 12-13 vessels/mm2, whereas in 50 measurements for each maceration sample. the remainder of the wood the frequency is on­ The average length of primary xylem vessel ly about 5-6/mm2 (Figs. 2, 3). members and fibres are based on 10-25 mea­ Moderately thin-walled fibres occur through­ surements from serial tangential sections. Ves­ out the wood (Figs. 1-3). They possess sparse, sel diameter and frequency were determined minute, simple pits. Although there is consider­ from the entire transverse sections, and the de­ able variation in fibre length (Fig. 13), the mean velopmental changes in ray structure were de­ length is rather uniform throughout secondary termined from the serial tangential sections. growth, averaging 625 J.1.m. Axial parenchyma With one exception, terminology used fol­ is para tracheal, scanty to vasicentric through­ lows the recommendations of the Committee out the secondary xylem (Figs. 2, 3). The thin­ on Nomenclature, lAW A (1964). In the case walled parenchyma consists mainly of fusiform of rays, initials of individual cells within a ray cells with a few strands of two cells. are called ray cell initials, whereas the entire At the beginning of secondary growth, pri­ group of such initials is called ray initial. This mary rays (Barghoorn, 1940) originate opposite modification of accepted terminology, first the medullary rays (Fig. I). These are similar to proposed by Cheadle and Esau (1964), is neces­ the medullary rays, being 2-3 cells wide and sary when describing the transformation of fu­ most extending the entire height of the sections. siform initials into ray initials. All cells are erect. Primary rays are absent in the fascicular areas of the xylem (Fig. I). During Results the formation of the first few mm of wood, the Primary xylem vessels occur in widely spaced primary rays increase in width by the conver­ strands surrounding the massive pith. These sion of fusiform cambial initials into ray cell strands are typically large and contain numer­ initials along the sides of the ray initials (Fig. 4). ous metaxylem vessels (Fig. I). The vessel mem­ These erect cells along the sides of the rays are bers show a fairly abrupt decrease in length, sheath cells, and are quite similar to fibres. As a from 700 to about 540 pm, progressing from result, the limits of the rays are difficult to de­ the inner to the outer metaxylem, and an in­ termine, particUlarly in transverse sections (Figs. crease in diameter from 47 to 90 pm (Fig. 13). 1, 2). At the same time that the rays are in­ In the central region between these strands are creasing in width, transverse divisions in ray additional strands consisting of wide, fibrous cell initials decreases the height of the ray cells. cells (Fig. I) with transverse or abruptly tapered While the primary rays are undergoing devel­ end walls.