Interxylary Phloem: Diversity and Functions Sherwin Carlquist
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Interxylary phloem: Diversity and functions Sherwin Carlquist Brittonia ISSN 0007-196X Brittonia DOI 10.1007/s12228-012-9298-1 1 23 Your article is protected by copyright and all rights are held exclusively by The New York Botanical Garden. This e-offprint is for personal use only and shall not be self- archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”. 1 23 Author's personal copy Interxylary phloem: Diversity and functions SHERWIN CARLQUIST Santa Barbara Botanic Garden, 1212 Mission Canyon Road, Santa Barbara, CA 93105, USA; e-mail: [email protected] Abstract. Interxylary phloem is here defined as strands or bands of phloem embedded within the secondary xylem of a stem or root of a plant that has a single vascular cambium. In this definition, interxylary phloem differs from intraxylary phloem, bi- collateral bundles, pith bundles, and successive cambia. The inclusive but variously applied terms included phloem and internal phloem must be rejected. Histological aspects of interxylary phloem are reviewed and original data are presented. Topics covered include duration of interxylary phloem; relationship in abundance between sieve tubes in external phloem and interxylary phloem; distinctions between interx- ylary and intraxylary phloem; presence of parenchyma, fibers, and crystals in the interxylary phloem strands; development of cambia within interxylary phloem stran- ds; three-dimensionalization and longevity of phloem, systematic distribution of int- erxylary phloem; physiological significance; and habital correlations. No single physiological phenomenon seems to explain all instances of interxylary phloem oc- currence, but rapidity and volume of photosynthate transport seem implicated in most instances. Key Words: Bicollateral bundles, included phloem, intraxylary phloem, photosyn- thate conduction, successive cambia. Interxylary phloem consists of strands of Chalk and Chattaway (1937), but used also for sieve tubes, companion cells, and adjacent instances of interxylary phloem. Misapplica- parenchyma or other cells embedded within tions of this sort render the term included the secondary xylem of a stem or root that has phloem imprecise, and, in any case, are based a single vascular cambium. This definition is on topographic phloem distribution without presented to distinguish interxylary phloem regard to ontogenetic factors. The ontogeny of from a series of other histological phenomena phloem and xylem within various cambia that may have similar functions but are variants can be easily determined from the histologically and ontogenetically different. For mature histology, and thus can readily be example, the term successive cambia denotes a includedindefinitions of cambial variants. The series of vascular increments, each with second- term "included" suggests that the phloem in ary phloem, secondary xylem, and a vascular instances of successive cambia is embedded cambium, each of which ultimately originates within secondary xylem (as it is in the case of from the master cambium at the periphery of a interxylary phloem). In fact, the phloem in stem or root (Carlquist, 2007). The master examples of successive cambia lies between cambium produces secondary cortex (0 paren- secondary xylem (internal to it) and conjunctive chyma) to the outside, and to the inside, tissue (external to it) in each vascular increment. conjunctive tissue and vascular cambia to the The term internal phloem has likewise been inside of an axis. Each vascular cambium then contaminated by conflicting usages and should produces secondary phloem to the outside and be rejected. "Internal phloem" has been used to secondary xylem to the inside. The term refer to intraxylary phloem, but has been included phloem was misapplied to successive applied to other histological conditions. Signif- cambia in some (but not all) Nyctaginaceae by icantly, like "included" phloem, the term Brittonia, DOI 10.1007/s12228-012-9298-1 ISSN: 0007-196X (print) ISSN: 1938-436X (electronic) © 2013, by The New York Botanical Garden Press, Bronx, NY 10458-5126 U.S.A. Author's personal copy BRITTONIA [VOL internal phloem is vague with respect to materials and methods used in those studies are ontogeny as well as location of phloem. given in the papers listed below. In all cases, Intraxylary phloem, although readily dis- however, the photographs and observations are tinguishable from interxylary phloem, may new. Re-studied materials include the following: have a similar physiological significance and Figure 1: Turbina stenosiphon (Hallier f.) is covered in a later section of this paper. The A. Meeuse (Convolvulaceae): Carlquist and present usages are consistent with those Hanson, 1991. adopted in earlier accounts of cambial var- Figure 2A–C: Thunbergia laurifolia Lindl. iants (Carlquist, 1988, 2001, 2007). For the (Acanthaceae): Carlquist and Zona, 1988 present, workers would be well advised to Figure 2D: Stylidium glandulosum Salisb. define the terms they use for cambial variants. (Stylidiaceae): Carlquist, 1981 Interpretation of functions of interxylary Figure 4: Pseudolopezia longiflora Rose phloem is further complicated by the fact that and Oenothera linifolia Nutt. (Onagraceae): in woody angiosperms as a whole, interxylary Carlquist, 1975. phloem occurs in only a relatively small Figure 6: Salvadora persica L.(Salvador- number of families and species. Even within aceae): Carlquist, 2002. a genus such as Combretum or Strychnos, Sources for material not previously studied some species have interxylary phloem, others are as follows: lack it, with no clear differences in habit or Figure 3: Orphium frutescens E. Mey. size of plant (van Vliet, 1979; Mennega, (Gentianaceae): Carlquist 8212, June 28, 1980). However, there is some correlation 2011 (SBBG). with systematic units within genera such as Figure 5: Craterosiphon scandens Eng. & these (van Vliet, 1979; Mennega, 1980). Gilg (Thymeleaceae): Breteler 1227 (WAG). – There is no unique function for interxylary Figure 7A B. Strychnos madagascariensis phloem; other phloem distributions seem to be Poir. (Loganiaceae): David Lorence 10285 adequate alternates. That does not mean, (PTBG), National Tropical Botanical Garden however, that interxylary phloem is not a living collections accession number 801348. – physiologically significant way of meeting a Figure 7C D. Combretum erythrophyllum plant's photosynthate conduction requirements. Sond. (Combretaceae): cultivated in the Wood anatomy contains many examples of Vavra Garden (formerly owned by University alternative ways of serving particular functions of California, Los Angeles). – (e.g., vestured pits, helical sculpture of vessel The sections in Figs. 3 and 7C D were surfaces, and vasicentric tracheids are probably derived from living material that was pre- all methods of minimizing embolism formation served in 50 % aqueous ethanol, sectioned on —or reversing that). Interxylary phloem, like a sliding microtome, and stained with a vestured pits, is a device that is homoplastic in Safranin-Fast Green combination. The section – woody angiosperms. In both instances, genetic in Fig. 7A B was derived from living information for the formation of these struc- material that was preserved in aqueous 50 % tures has not been frequently achieved phylo- ethanol. The sections were prepared by driving genetically, perhaps because a complex series a single-edged razor blade into a stem with the of genetic changes is required. The data aid of a hammer. The sections derived were presented here may offer interesting examples subjected to changes of distilled water, and that lend themselves to physiological studies. dried between glass slides under pressure (to Plant physiological studies have traditionally prevent curling), then sputter-coated with gold been done on economically important plants, and examined with a Hitachi S2600N scanning and none of the species with interxylary phloem electron microscope (SEM). has any major economic value. Aspects of Interxylary Phloem 1. Ontogenetic and Histological Criteria; Materials and methods Allied Phenomena. Some of the examples cited here are derived Because secondary xylem consists mostly from earlier wood anatomical surveys. The of cells with rigid walls, it is a clear and Author's personal copy 2013] CARLQUIST: INTERXYLARY PHLOEM FIG.1. Transections of stem of Turbina stenosiphon (Convolvulaceae), to show diverse types of vascular histology. A–B. Successive cambia. A. Three vascular increments, each with secondary xylem (sx) and secondary phloem (sp); the middle vascular increment has an inverted orientation (ivi), atypical for successive cambia. B. Higher power, area corresponding to center of A. The inverted increment (ivi) above has produced secondary phloem adaxially and secondary xylem abaxially; the crushed secondary phloem (csp) was produced by the inverted increment. The normal increment below has produced secondary phloem abaxially and secondary xylem adaxially. C. Strand of interxylary phloem, surrounded by fibrous secondary xylem.