The Vascular System of Monocotyledonous Stems Author(s): Martin H. Zimmermann and P. B. Tomlinson Source: Botanical Gazette, Vol. 133, No. 2 (Jun., 1972), pp. 141-155 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/2473813 . Accessed: 30/08/2011 15:50 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to Botanical Gazette. http://www.jstor.org 1972] McCONNELL& STRUCKMEYER ALAR AND BORON-DEFICIENTTAGETES 141 tomato, turnip and cotton to variations in boron nutri- Further investigationson the relation of photoperiodto tion. II. Anatomical responses. BOT.GAZ. 118:53-71. the boron requirementsof plants. BOT.GAZ. 109:237-249. REED, D. J., T. C. MOORE, and J. D. ANDERSON. 1965. Plant WATANABE,R., W. CHORNEY,J. SKOK,and S. H. WENDER growth retardant B-995: a possible mode of action. 1964. Effect of boron deficiency on polyphenol produc- Science 148: 1469-1471. tion in the sunflower.Phytochemistry 3:391-393. SKOK, J. 1957. Relationships of boron nutrition to radio- ZEEVAART,J. A. D. 1966. Inhibition of stem growth and sensitivity of sunflower plants. Plant Physiol. 32:648-658. flower formation in Pharbitis nil with N, N-dimethyl- STRUCKMEYER, B. ESTHER, and ROBERT MACVICAR. 1948. amino succinamicacid (B995). Planta 71:68-80. BOT.GAZ. 133(2) :141-155. 1972. THE VASCULARSYSTEM OF MONOCOTYLEDONOUSSTEMS MARTIN H. ZIMMERMANN AND P. B. TOMLINSON Harvard University}Cabot Foundation,Petersham, Massachusetts 01366 ABSTRACT The course of vascularbundles and its developmentalpattern in monocotyledonshas been reinvesti- gated. This shows the existenceof an "inner"and an "outer"vascular system. The inner system is more extensive, three-dimensionallyvery complex, and "open-ended"in a distal direction,i.e., centripetally toward the apical meristem.The outer system is open-endedin basal and peripheraldirections and usually poorly developed. In many monocotyledons,it is representedby the fibrous bundles in the cortex of the stem. Occasionally,the outer system is more fully developed as in Strelitzia, where it is representedby fully developed-vascular cortical bundles or as a secondary vascular tissue in those monocotyledonswhich show secondarygrowth (e.g., Dracaena,Cordyline, Pleomele). We suggest that dicotyledonsdiffer from monocotyledonsin having only an outer system. This might lead to a clarifica- tion of the phylogeneticrelationship between the two groups. Introduction activity is superficially similar to that of these Among the many structuralfeatures in which monocotyledons, because discrete conducting monocotyledonsdiffer from dicotyledons, prob- strands are produced within secondary tissue. ably the most constant and distinctive is stem Finally, there are a few monocotyledons with the anatomy.Monocotyledons usually have individual bundles in a single ring as in aerial stems of primaryvascular bundles containingboth phloem Dioscorea, which is very specialized, or small and xylem "scattered"throughout a single trans- plants as in the Mayacaceae, Petrosaviaceae, and versesection of the stem. In addition,most of them certain Eriocaulaceae ( Tonina), which are prob- lack secondary growth. In dicotyledons,on the ably simplified by reduction. other hand, primarybundles, where they are dis- The vascular architecture of a large number of crete, are usually in a single ring; but secondary dicotyledons is known in detail, and a considerable growth eventually produces the typical dicotyle- understanding of their pattern of development has donousstem with a xylem core, enclosedin a thin been achieved (cf. ESAU 1965). On the contrary, cylinder of phloem. There are minor exceptions monocotyledonous vascular systems have, until re- to this generalization.Some dicotyledons (e.g., cently, largely eluded our understandingbecause of species in the Amaranthaceae,Nyctaginaceae, their great complexity, which is largely beyond the Piperaceae) have more or less scattered primary reach of orthodox methods of investigation. The bundles. There are a few monocotyledons(e.g., crux of the problem lies in a study in precise detail Cordyline, Dracaena, and Yucca) with a vascular of how the monocotyledonous vascular system cambiumwhich producessecondary vascular bun- originates in the apical region. New methods had dles within secondaryground tissue. Furthermore, to be developed in order to make this possible. thereare dicotyledons(e.g., certainAmaranthaceae During the past few years we have studied the and Chenopodiaceae)in whichthe modeof cambial stems of many large monocotyledons, starting with the palms and later extending our observations to 1 The work leading toward this publication has been other families ( ZIMMERMANNand TOMLINSON supportedby a grant from the National ScienceFoundation 19 6 5, 19 6 7, 19 68, 19 69; TOMLINSONand ZIMMER- ( GB-5 762-X) to one of us (P. B . TOMLINSON). MANN1966a, 1966b, 1968a, 1968b). Our investiga- BOTANICALGAZETTE [JUNE 142 tionshave revealed a previouslyunrecognized pattern progress could be made only if we developed which we believe to be fundamentalfor mono- methods which enabled us to "find our way" cotyledonousstems as a whole.We believe that we through the maze of vascular tissue which the have also recognizedthe developmentalprinciple coconutstem represents.Our methodsnow permit which underlies this structuralpattern and can this kind of analysis. thereforesuggest the way in whichthis principlein Early we conceived the idea of putting indi- monocotyledonsdiffers from that in dicotyledons. vidual images of transverse sections onto indi- In addition, we believe that some of the more vidual frames of motion-picturefilm. This has constantfeatures of monocotyledonousmorphology becomeour mainmethod of structuralanalysis. We can be explainedby this principle.This may have found later that this basic idea was not new important consequencesin taxonomic interpre- (POSTLETHWAIT1962 ) . However, for reasons to tation and ultimatelyin understandingthe phylo- be explainedbelow, the methodhad not previously geny of monocotyledons. become a significant research tool. Structural A particularproblem is that of communicating analysis with frame-by-framecinematography to our results. The monocotyledonousvascular pat- provide new information can now be accom- tern is so complexthat it is difficultfor anyone plished in two differentways. to comprehendit who is not dealing with it di- A1OTION-PICTUREANALYSIS BY THE SURFACE rectly and studyingit thoroughly.Yet, if the prin- METHOD.The first method is to photographdi- ciple of growthand resultingstructure is as fun- rectly the surfaceof a specimenplaned on a micro- damentalas we think, we feel that we shouldmake tome.If this is done frameby framewith a motion- an effort to present it to nonspecialistsin a gen- picture camera,serial or sequentialimages can be eralizedand easily understoodform. The present storedfor subsequentanalysis. This had been done paperattempts to do this. Our presentation,there- by otherworkers in the past, but the methodwas of fore, becomesthat of a workinghypothesis. Our limited value because it was restricted to very observationshave shownthat the vascularpatterns short sequencesand dealt only with small objects of monocotyledonousstems representa series of which could be mounted on a rotary microtome variations on a basic theme. At this stage, we (POSTLETHWAIT1962 ) . In all existingmicrotomes, should like to emphasizeand simplify the theme the clamp holding the specimenis advancedbe- without saying too much about the variation.We cause it gives the most preciseadvance. EIowever, appreciatethat an enormousamount of additional for the largeobjects we have to analyze,we do not work still has to be done, but it will be easier to requirevery preciseadvance, and it is not neces- proceed with broad comparative investigations sary to cut thin sections (which are usually dis- once the fundamentals,or what we believe to be cardedanyway). For our analyses,it is necessary the fundamentals,are more widely comprehended. to cut a long specimencontinuously without having to reclampit. For this reason,the specimenitself Methods has to be advanced.This has been accomplished A descriptionof methodsbecomes necessary in with a specially designed "continuous-advance" the presentpaper, because the numericalcomplex- clamp which we use on a Reichert"OME" sliding ity of monocotyledonousvascular systems called microtome. The specimen is advanced through for drasticallynew procedures.Classical methods rollers in two possible ways. The rollers them- of plant anatomyare inadequateto deal with the selves, driven through a gearbox by hand, may large numberof vascular strands encounteredin advance the specimen.This arrangementis best the stems of large monocotyledons.For example,a suited to firm specimenswith straight sides. A1- transversesection of a coconut palm stem con- ternatively, a jack pushes the specimen, from tains about 20,000 central vascular bundles and below, throughthe rollers which then move pas- tens of thousandsof