Amer. J. Bot. 59(6): 573-586. 1972. THE MORPHOLOGY AND RELATIONSHIPS OF DALECHAMPIA SCANDENS (EUPHORBIACEAE)1 GRADY L. WEBSTER AND BARBARA D. WEBSTER Departmentof Botanyand Departmentof Agronomyand Range Science, Universityof California,Davis A B S T R A C T The circumtropicalbut preponderantlyAmerican genus Dalechampia, comprisingnearly 100 speciesof twiningvines (or rarelysubshrubs), is strikinglyisolated within the Euphorbiaceae because of its distinctivebibracteate inflorescences. There has been considerabletaxonomic controversywith regardto the relationshipsof the genus, and it has been suggestedthat Dalechampia is allied to the tribeEuphorbieae because of a supposed analogy betweenits inflorescenceand the cyathiumin the Euphorbieae. Field and laboratoryinvestigations of the commonAmerican species D. scandens,together with a comparativesurvey of related species,have thrownsome lighton these problems. The Dalechampia inflorescenceseems best interpretedas consistingof a terminalstaminate pleiochasium (with part of the lateral branchestransformed for nectarproduction), juxtaposed to a 3-floweredpistillate cyme. The lips of the conspicuousbilabiate involucre are formedby the hypertrophiedbracts which sub- tend the staminateand pistillatecymes. The bisexual inflorescencesappear to be distinctly proterogynous,rather than proterandrous, as has been previouslysuggested. The configuration of the inflorescence-abilaterally symmetrical pseudanthium-suggests adaptation for cross- pollination,but the closing movementof the bracts makes self-pollinationprobable in the absenceof visitsby pollinators.The similarityof theDalechampia inflorescence to thecyathium of the Euphorbieaeappears to be entirelysuperficial, and both reproductiveand vegetative data suggestthat Dalechampia is relatedto taxa of tribePlukenetieae. DALECHAMPIA IS A GENUS of Euphorbiaceae so withother taxa of Euphorbiaceaehave been con- isolated and sharplydelimited that it has usually troversial.Mueller (1866) placed the tribeDale- been placed in a distinctmonogeneric tribe since champieae adjacent to the Euphorbieae. He thus the originaldisposition of Mueller (1864). Ac- adoptedan interpretationsimilar to thatof Jussieu cordingto Pax and Hoffmann(1931), the genus (1824), who referredDalechampia to his Sectio comprisesabout 100 tropical species, of which VI along withAnthostema, Euphorbia, and Pedi- over three-fourthsare American;probably half of lanthus. Baillon (1858), on the otherhand, re- the species in the genus occur withinthe bound- jectedthis disposition (though partly for the wrong aries of Brazil. Most of the common species of reasons, since he regardedthe cyathiumof Eu- Dalechampia are herbaceous twiningvines, but phorbiaas a hermaphroditicflower) and indicated some of the Brazilian ones are suffrutescent,and an affinityof Dalechampia to Plukenetia.Bentham one of these-D. spathulata-is commonlyculti- (1878, 1880) went so far as to include Dale- vated in greenhousesbecause of its attractivered- champiain the subtribePlukenetiinae, adjacent to dish inflorescences. Tragia and Cnesmone. Pax (1890) at firstmade The systematicproblems facing the studentof the same dispositionas Bentham,but later (Pax Dalechampia are somewhatdifferent from those and Hoffmann, 1919, 1931) removed Dale- usually encounteredin the Euphorbiaceae,where champia to a separate tribe without,however, the major difficultylies in definingand discrimi- negatingthe relationshipto the Plukenetieae(here nating among taxa. Dalechampia is so sharply treated as a tribe in accord with Hutchinson, differentiatedfrom other Euphorbiaceous taxa and 1969). at the same time so relativelyhomogeneous at More recentlyCroizat (1940, 1942) has re- the infragenericlevel thatits genericintegrity has turnedto the originalhypothesis of Jussieu(1824) never been seriouslydoubted. Preciselybecause and has attemptedto derive the cyathiumof of its isolated position,however, its relationships Euphorbiafrom the Dalechampia inflorescence via 1 Receivedfor publication2 September1971. the zygomorphiccyathium of Pedilanthus. How- The authorsare pleased to acknowledgethe assistance ever, in his monographof Pedilanthus,Dressler of Dr. Kim I. Miller,Mr. Bryce Christman,and Miss (1957) has remarkedthat there is no evidenceto Mary Craig in the preparationof this paper. Part of the researchwas supportedby grantsfrom the National supportthis theory. Hurusawa (1954), on the Science Foundation. otherhand, has followedCroizat in placingDale- 573 This content downloaded from 169.237.8.35 on Mon, 16 Sep 2013 16:48:05 PM All use subject to JSTOR Terms and Conditions 574 AMERICAN JOURNAL OF BOTANY [Vol. 59 champia adjacent to the Euphorbieaeand Antho- relativelyancient one, and not the resultof recent stemeae. long-distancedispersal. At any rate, it should be The interpretationof the Dalechampia inflores- kept in mind that the discussionsin this paper cence is of considerable systematicimportance, apply only to the NorthAmerican populations of since it may affectnot only the placementof this D. scandens and do not take into account varia- genusbut the tribal arrangements within the family tions which mightoccur in the South American as a whole. There are furthermoresome purely or Africanplants. morphologicalproblems with regard to the in- florescencewhich need to be settledbefore mak- MATERIALS AND METHODS-Living plants of ing a systematicjudgment. Urban (1888) took Dalechampia were studiedin the greenhousesat issue with the interpretationof Baillon (1858) the Universityof California,Davis, and at Purdue that the secretoryscales of the male part of the University,Lafayette, Indiana. Wild populations inflorescencerepresent modified bracts of sterile were observedin Mexico, Nicaragua, and in the cyme-branches.Because therewere no bractsas- West Indies. sociatedwith the male flowersin the inflorescence Anatomicalpreparations were made fromma- of D. spathulata2which he investigated,Urban terialfixed in FAA (fornLalin,acetic acid, alcohol), proposed that the scales representedmodified dehydratedthrough the standard TBA (tertiary flowers. Michaelis (1924), on the basis of a butylalcohol) series,embedded in paraffin,and surveyof a largenumber of species,disagreed and stained followingthe usual safranin-fastgreen returnedto an interpretationcloser to that of technique(Jensen, 1962). Baillon. Macrophotographswere taken with polaroid Aside fromthe intrinsicallyinteresting features film,using an Aristophotapparatus. Photomicro- of this curious genus,we wish to contributeto a graphsof sectionedmaterial were also takenwith better understandingof its relationshipsby re- polaroidfilm, using a Zeiss photomicroscope.The investigatingits morphologywith particularem- photographsof pollen and trichomeswere made phasis on inflorescenceand floral structure. with a CambridgeStereoscan Microscope; speci- Relativelyfew species of Dalechampia are found mens were treatedwith gold shadowing. in cultivation,and theonly detailed morphological Voucherspecimens of Dalechampia plantshave analysis of the genus (Michaelis, 1924) omits been deposited in the Department of Botany considerationof most structuresother than the Herbarium,University of California,Davis. inflorescence.It thus appears sound strategyto The terminologyused here is that of Webster thoroughlydefine the structureand morphological (1968), where a detailed taxonomicdescription variationswithin a singlespecies. may be found. The species consideredhere, D. scandens,pre- sentsa unique problemsince it is the only species RESULTS-Growthand phyllotaxy-Dalecham- in the genus listedas nativeto both America and pia scandens,like the otherspecies of sect. Scan- Africa(Pax and Hoffmann,1919). It is notclear dentes (Pax and Hoffmann,1919), is an herba- whethertheir dispositionwill stand the test of ceous, twiningvine which in natureeither climbs contemporaryinvestigations, but the apparent or sprawlsalong the ground. The aerial portion presenceof distinctendemic races in both South of the plant is composed of twininglong-shoots America and Africa suggests a natural trans- which come off the suffruticosebase and which Atlantic occurrence of D. scandens. Leandri may die back duringunfavorable seasons. The (1943), for example,has recognizedD. scandens characteristically3-lobed leaves are borne on the as nativeto Madagascar,and has pointedout close monopodiallong-shoots in a spiral arrangement; affinitiesbetween it and some of the endemic each leaf on the long-shootmay subtenda short- Malagasian species. Leonard (1962) appears to shoot,which terminates in an inflorescence(Fig. accept some African forms of Dalechampia as 1). Each short-shootmay produce a single in- belongingto D. scandens. It is possible, there- florescencesubtended by a "rameal" leaf smaller fore, that the distributionof D. scandens is a than thatfound on the long-shoot.On the other hand, the rameal leaf may subtenda second in- 2 Urban used the name D. roezliana Muell. Arg. for volucre with its own subtendingrameal leaf. thisplant, and the same name has been appliedin more Technically,the inflorescencesshould perhapsbe recentpapers by Michaelisand VenkataRao, despitethe fact that Pax and Hoffmann(1919) have relegatedD. termedpseudoterminal, since there is nearly al- roezliana to synonymyunder D. spathulata (Scheidw.) ways a bud in the axil of the firstrameal leaf, and Baill. thisprobably represents the terminalbud. Unlike Fig. 1-4. Habit of Dalechampiascandens.-Fig. 1. Portionof long-shoot,with axillaryshort-shoot bearing an inflorescenceand small rameal