Ant-Pollination of the Granite Outcrop Endemic Diamorpha smallii () Author(s): Robert Wyatt Reviewed work(s): Source: American Journal of Botany, Vol. 68, No. 9 (Oct., 1981), pp. 1212-1217 Published by: Botanical Society of America Stable URL: http://www.jstor.org/stable/2443043 . Accessed: 01/01/2013 09:57

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].

.

Botanical Society of America is collaborating with JSTOR to digitize, preserve and extend access to American Journal of Botany.

http://www.jstor.org

This content downloaded on Tue, 1 Jan 2013 09:57:50 AM All use subject to JSTOR Terms and Conditions Amer. J. Bot. 68(9): 1212-1217. 1981.

ANT-POLLINATION OF THE GRANITE OUTCROP ENDEMIC DIAMORPHA SMALLII (CRASSULACEAE)l

ROBERT WYATT Departmentof Botany, University of ,Athens, Georgia30602

ABSTRACT Formicaschaufussi, a medium-sizedant commonthroughout the Piedmontand CoastalPlain of the southeasternUnited States, has been observed in association with Diamorphasmallii, a self-incompatibleannual endemic to granite outcrops in the Southeast, over a 6-yr period. Observationsof ants collected on the flowers show large numbersof the sticky pollen grains adheringto hairs and indentationsprimarily on the thoraces of the ants. Thoughsmall native bees and flies are frequent and sometimes effective pollinators,it is highly unlikely that the monotypicgenus Diamorphacoevolved with the introducedhoneybee, as has been suggested. Characteristicsindicative of an "ant-pollinationsyndrome" include occurrence in hot, dry habitats, a high density of very small bearingoverlapping inflorescences at a uniform height, and low seed number,pollen volume, and nectar quantity.

IT IS ONLY RECENTLY, withincreased emphasis agreed with Hagerup(1943) that ant-pollinated on -animalinteractions (e.g., Faegri and are most likely to be annual plants of van der Pijl, 1971; Proctor and Yeo, 1972; hot, dry habitats. This paper reports a case of Heinrichand Raven, 1972;Gilbert and Raven, pollination of Diamorpha smallii Britton 1975), that the phenomenon of pollination by (Crassulaceae),a succulent annualendemic to ants, dismissed by classical pollination ecol- granite outcrops in the southeastern United ogists (Kerner, 1878; Muller, 1883), has been States, by Formica schaufussi Mayr (Hyme- thoroughly described and adequately docu- noptera: Formicidae), a medium-sized ant mented. Hickman(1974) was the first to report common in the Piedmont and Coastal Plain of effective pollination of the flowers of Polyg- the Southeast. onumcascadense Baker(Polygonaceae) by the ant Formica argentea Wheeler (Formicidae). MATERIALS AND METHODS-Field obser- Although he could not prove the absolute de- vations of visitors to the flowers of Diamorpha pendence of the plants on ants for pollination, smallii were initiatedin 1975and continuedon Hickman presented strong circumstantialevi- a regular basis through 1980. Plant vouchers dence includingobservation of very few visits from the site where ants were first observed to the flowers by other pollinatorsin the field in RutherfordCounty, (Wyatt, and observation of greatly reduced fruit-set in 1977;Wyatt and Fowler, 1977),are in the her- the absence of the ants in the greenhouse. bariumof Duke University (DUKE): Wyattet Hickman(1974) was able to extrapolatefrom al. 612, Wyatt 746, 747, 748. Insect visitors the characteristicsof the specific Polygonum- also were observed and captured either with Formica system to the traits necessary for an nets or aspiratorson granite outcrops in Ala- "ant-pollination syndrome." Applying these bama, Georgia, and . A refer- criteria to six earlier reports of pollinationby ence collection of insects identifiedby experts ants, Hickman concluded that four of these at the USDA Insect IdentificationInstitute in were highly probable, while the remainingtwo Beltsville, Maryland,was establishedand used were merelypossible or unlikelyexamples. He to identify insects capturedon the flowers. In order to investigate whether or not ants I Received for publication 8 August 1980; revision ac- were transportingpollen as they moved about cepted 21 November 1980. on the inflorescences of Diamorpha, 20 ants I thank J. Antonovics, R. H. Chapman, J. C. Hickman, were captured and later examined for pollen R. S. Peigler, and R. L. Wilbur for comments on an earlier under a dissecting microscope (30x magnifi- draft of the manuscript. Assistance with field work was provided by M. Caisse, N. Fowler, T. R. Meagher, R. B. cation). Ants were also observed with a scan- Primack, A. Stoneburner, A. H. Teramura, and K. Ter- ning electron microscope (SEM). Specimens amura. Identification of insect pollinators was done by L. were coated with approximately20 nm of gold- Knutson and his staff at the USDA Insect Identification palladiumand mountedon aluminumstubs by Institute: S. W. T. Batra, R. J. Gagne, J. M. Kingsolver, was car- D. R. Smith, G. Steyskal, and F. C. Thompson. Support means of double-sticktape. Viewing from NSF Grant No. DEB-8001986 helped make this re- ried out with a JEOL JSM-35 SEM operated search possible. at an acceleratingvoltage of 15 kv. 1212

This content downloaded on Tue, 1 Jan 2013 09:57:50 AM All use subject to JSTOR Terms and Conditions October, 1981] WYATT-ANT-POLLINATION OF DIAMORPHA 1213

On 17 March 1979, populationsof Diamor- nae of any ant, possibly due to grooming of pha smallii were collected from Heard County, those parts (Fig. 5). Georgia (Flat Rock Campgroundon GA 34, 6 Other insects captured on the flowers of km SW of Franklin; population "L" of Diamorpha in the Carolinas and Georgia in- McCormick and Platt, 1964), and Randolph cluded honeybees and small native bees, flies, County, (Bald Rock, 0.3 km S of butterflies,and a pollen-eatingbeetle. Little or Almond off AL 77; population "N" of Mc- no pollen was present on the flies or butterflies, Cormickand Platt, 1964).Plants were collected but the bees carriedconsiderable pollen. Their in the rosette stage, and they initiatedbuds and behavior, especially that of the larger honey- flowered under natural light in a greenhouse bees, was erratic. They typically visited one in College Station, Texas. The plants were or two flowers in an area and then flew some maintainedas intact mats in their naturalsub- distance away before again alighting. Appar- strate in flats suspended in closed containers ently the bees are potentially effective polli- of demineralized water. The plants of both nators, especially over long distances, but they populationswere in full flower by 10 April, 1- were never present in large numbers during 2 wk in advance of the floweringtime observed our observationperiods. Hymenopteranscap- by McCormickand Platt (1964)for these same turedincluded Apis melliferaL. of the Apidae, populationstransplanted to Mt. Arabia, Geor- a species of Dialictus near D. imitatus (Sm.) gia. and a species of Sphecodes in the Halictidae, Crosses were performedunder a dissecting and two species of Andrenain the Andrenidae. microscope (30x magnification)using forceps Among the Diptera were Eristalis arbustorum and dissecting needles. Four flowers from (L.) and Eristalinus aeneus (Scopoli) of the Georgia were selfed; four were crossed with Syrphidae, Phaenicia sericata (Meigen) of the plants from within that population; and four Calliphoridae,and Hylemya platura (Meigen) were crossed with plants from the Alabama of the Anthomyiidae.The two Lepidopterans population. Approximately 50 pollen grains captured were Precis coenia Hiibner of the were deposited on each of the four stigmas of Nymphalidae and Callophyrs gryneus (Hub- each flower crossed. These were later har- ner) of the Lycaenidae. The pollen-eatingCo- vested and scored for success or failure of leopteran was Collops tricolor Say of the Me- seed-set. lyridae. Two of the four within-populationcrosses REsULTs-Ants were first observed on the set a full complement of seeds, but only one flowers of Diamorpha smallii on 7 May 1975 of the four between-populationcrosses was at a granite outcrop in North Carolina. The successful. None of the self-pollinations re- ants moved rapidlyfrom flower to flower seek- sulted in seed-set, suggesting that D. smallii ing nectar produced at the bases of the petals is self-incompatible.Furthermore, none of the and were extremely abundanton the flowers thousands of plants grown under greenhouse throughoutthe day. At any one time an average conditions with all pollinators excluded pro- of 15-20 ants were present within a shallow duced seeds. Under naturalconditions plants depression approximately 0.5 m2 that was typically produce a full complement of 12 watched intensively. Subsequent visits to the seeds. This suggests that the activity of polli- same locality on 16 May 1978, 17 May 1979, nators is absolutely necessary for seed pro- and 15 May 1980 revealed large numbers of ductionin Diamorpha. Comparisonof fruit-set ants on the flowers of Diamorpha. Visits to in populations under natural conditions with other outcropsin Alabama,Georgia, and South those from which all pollinatorsbut ants were Carolinasimilarly showed the frequent occur- excluded would be useful in allowing direct rence of Formica schaufussi in association quantification of the relative importance of with these plants. Formica subsericea Say was these insects as pollinators of these plants. also observed on Diamorphaflowers in South Carolina. DIscussioN-The of Diamorpha Pollen loads on the ants varied from light smallii has been a matter of debate for over (a few scattered grains) to heavy (numerous 150 years. Nomenclatural arguments have clumps of 5-20 grains each). Pollen was most been summarizedby Wilbur(1964, 1977),Clau- abundant in the indentations between body sen (1975), and Spongberg(1978). Some recent segments and along the legs (Fig. 5, 6). Hairs authors(e.g., Ahles, 1964;Radford, Ahles, and located at the rear of the head and on the ab- Bell, 1964;Sharitz and McCormick,1973) have domen sometimes held pollen grains (Fig. 5), suggested that this monotypic is better but pollen was most abundanton the thorax. treated as a species of , its closest rel- No pollen grains were observed on the anten- ative in the Crassulaceae. McCormick and

This content downloaded on Tue, 1 Jan 2013 09:57:50 AM All use subject to JSTOR Terms and Conditions 1214 AMERICAN JOURNAL OF BOTANY [Vol. 68

Platt (1964) reported naturaland artificialhy- 8) Seeds are relatively few per flower. Seed bridsproduced by crossing of this species with number was calculated as 12.0 + 0.01 for Dia- Sedumpusillum Michaux,another granite out- morpha (Sharitz and McCormick, 1973), a val- crop endemic. However, no specimens exist ue in agreement with greenhouse crosses dem- to support these observations (Sherwin and onstrating three seeds produced by each of the Wilbur,1971). Furthermore, in view of the fact four follicles of a single flower. Theoretically, that Sherwinand Wilbur(1971) have described the transfer of only 12 pollen grains could result a number of major anatomical features that in production of a full complement of seeds. clearly separate the two genera, their recog- 9) Flowers are small with the immature red nition of Diamorpha as a distinct genus is fol- anthers contrasting with the white petals. The lowed here. Clausen (1975), Chapman(1977), plant body itself is red, contrasting with the and Spongberg(1978) also have followed this petals and presenting quite a spectacular show approach. for the human observer. A faintly sweet odor Diamorpha smallii fits almost perfectly the is detectable from large masses of the flowers. ten traits characteristic of ant-pollinated 10) Nectar quantity appears to be quite species (Hickman, 1974): small and unlikely to attract pollinators with 1) It grows on granite outcrops which are high energy demands. characterized by high temperaturesand low The consistency with which the ants and water availability(Fig. 1). Although the Pied- plants co-occur in space and time initially sug- mont of the Southeast is generally an area of gested a biological relationship. This has now moderately high rainfall, the outcrops them- been confirmed by direct observation of pol- selves present a severe habitatfor many kinds linator movement patterns between plants in of plants, which show peculiar adaptationsto the field and by SEM and light microscopic these unique habitats (Keever, 1957; Cotter demonstration of Diamorpha pollen on the and Platt, 1959; Wiggs and Platt, 1962; Mc- ants. The observation in this study that D. Cormick and Platt, 1964; Ware, 1972; Baskin smallii is self-incompatible concurs with the and Baskin, 1972; Sharitz and McCormick, conclusion of Chapman (1977) that the species 1973). is obligately outcrossed. In that study, seed- 2) Nectaries are readily accessible to small set from 143 self-pollinations was less than 1% insects with short tongues (Spongberg, 1978). of that obtained from 98 cross-pollinations. They are located at the base of each of the four The taxonomy of Formica schaufussi has petals between the carpels and antipetalous been nearly as confused as that of Diamorpha stamens. smallii. Although the species has often been 3) The plants are extremely small (Fig. 2). treated as a subspecies of F. pallidefulva La- They average about 4 cm in height but may treille composed of a number of varieties (e.g., flower when only 2 cm tall. Under favorable Wheeler, 1913; Dennis, 1938; Cole, 1940; conditionsthey may reach 7-9 cm (McCormick Smith, 1947, 1951), the taxonomy followed and Platt, 1964). here is that of Creighton (1950). By his tax- 4) Populationsof Diamorphaare extremely onomy, the ants collected on Diamorpha are dense with a high degree of overlap of neigh- referable to Formica schaufussi ssp. dolosa boringplants (Fig. 3). Sharitzand McCormick Wheeler. These ants occur from central Texas (1973)reported average densities of 41.2 + 0.56 east to Florida and northeastward to southern (x + s) plants per dM2. In favorable situations (Creighton, 1950). Therefore, the of low density the plants may branchprofusely range of F. schaufussi corresponds well with from the base, which maintainsa high effective that of granite outcrops in the Southeast, and density of flowers. the species has been reported from all parts of 5) Synchronouslyblooming flowers are rel- the range of D. smallii: Georgia (Wheeler, atively few per plant. Plants bear an average 1913), North Carolina (Wheeler, 1904), South of 9.5 flowers (Wiggs and Platt, 1962) with Carolina (Smith, 1934), and (Den- some overlap in blooming time of flowers on nis, 1938; Cole, 1940). an individual plant. Movement of pollen be- Formica schaufussi occurs in moderately tween plants is virtuallyassured, nevertheless, large colonies at lower elevations (Wheeler, by the extremely high density and degree of 1913). Open sites are preferred for nesting spatial overlap of adjacentplants (Fig. 4). either in dry grassland or early successional 6) Flowers are borne on short pedicels at a pine communities (Cole, 1940). The ants typ- uniform height on stiffly erect or ascending ically live in the ground but will climb trees to plants. forage for honey dew (Dennis, 1938). Wheeler 7) The volume of pollen produced appears (1913) suggests that the species subsists largely to be relatively small. Also, pollen grains are on dead insects and the excreta of aphids. large and tend to cohere in clumps (Fig. 7). These ants are extremely agile but quite timid

This content downloaded on Tue, 1 Jan 2013 09:57:50 AM All use subject to JSTOR Terms and Conditions October, 1981] WYATT-ANT-POLLINATION OF DIAMORPHA 1215

1_44

m u~ ~~~~~~~~~~~~~~~~~~~-4-

Fig. 1-7. 1. Diamorpha smallii population growing in a shallow soil depression over granite. The diameter of the depression is about 0.5 m. 2. Plants of D. smallii indicating the extremely high density within the population. The plants are about 4 cm tall. 3. Formica subsericea on flowers of D. smallii. Note the dense overlapping of flowers on adjacent plants forming a canopy of uniform height. The flower is about 3 mm in diameter. 4. Closer view of F. schaufussi on flowers of D. smallii. The ant moves from flower to flower seeking nectar from the bases of the antipetalous stamens. The ant is about 5 mm long. 5. SEM of head of F. schaufussi. Note the many hairs of the ant and large clumps of pollen of D. smallii (x58). 6. SEM view of pollen of D. smallii on thorax of F. schaufussi (x478). 7. Closer SEM view of pollen of D. smallii on F. schaufussi (xI,047).

This content downloaded on Tue, 1 Jan 2013 09:57:50 AM All use subject to JSTOR Terms and Conditions 1216 AMERICAN JOURNAL OF BOTANY [Vol. 68

(Cole, 1940)and are enslaved by various taxa ants may have an impact on the delicate bal- of Polyergus and the Formica sanguinea ance between nectarproduction and pollinator group (Smith, 1947). service (Heinrich and Raven, 1972). Barrows Morphologically, F. schaufussi ssp. dolosa (1976) demonstrated beneficial effects from is suitable as a pollinator because of its long enhanced pollinator movement between flow- and dense pubescence (Cole, 1940), a feature ers of the self-incompatibleLantana camara used taxonomicallyto separate it from related L., whereas Wyatt(1980) showed that ants had conspecific taxa (Fig. 5, 6). The feeding habits a negative effect on effectiveness and efficien- and preferrednesting sites of this ant also con- cy of pollination of Asclepias curassavica L. tribute to its behavioral suitability for polli- This suggests that relationshipsbetween ants nation of small granite outcrop annuals such and flowers on plants may span the full spec- as Diamorpha smallii. trum of beneficial and harmfuleffects. These It remains to be determined, however, to range from harmful nectar-robbingthat re- what extent the relationshipbetween ants and duces pollinatorservice, throughpromotion of D. smallii is the product of coevolution. The pollinatorservice and enhanced levels of out- few honeybees and flies observed in natural crossing, to effective pollen dispersal by the populations do not appear to be major polli- ants themselves. There is a great need for ex- nators of Diamorpha; however, in years un- perimentalinvestigations of the precise costs usually favorable for growth where abundant and benefits associated with attraction or re- moisture delays flower maturation,these pol- pulsion of ants by flowers along this continuum linators may become more prominent. Their of possible interactions. importance similarly may vary from outcrop to outcrop. Chapman (1977) concluded that LITERATURE CITED honeybees are the major pollinators of D. smallii on outcrops in Georgia and that most AHLES,H. E. 1964. New combinations for some vascular plants of southeastern . J. Elisha Mitch- pollinatorflights are short-rangeones between ell Sci. Soc. 80: 172-173. flowers within small areas. That the pollination BAKER, H. G., AND I. BAKER. 1975. Studies of nectar system represents coevolution of the plants constitution and pollinator-plant coevolution. In L. with honeybees seems very remote, since this E. Gilbert and P. H. Raven [eds.], Coevolution of species was introducedless than 300 years ago. animals and plants, p. 100-140. University of Texas Because the granite outcrops of the Southeast Press, Austin. , AND . 1978. Ants and flowers. Biotropica and their endemic species are probably quite 10: 80. ancient (McVaugh, 1943;Burbanck and Platt, BARROWS, E. M. 1976. Nectar robbing and pollination 1964; Murdy, 1968; Palmer, 1970; Wyatt and of Lantana camara (Verbenaceae). Biotropica 8: 132- Fowler, 1977), the unique morphologyand re- 135. productive characteristics of D. smallii are BASKIN, J. M., AND C. C. BASKIN. 1972. Germination unlikely to have evolved in association with characteristics of Diamorpha cymosa seeds and an ecological interpretation. Oecologia 10: 17-28. honeybees. The most likely candidates, other BURBANCK, M. P., AND R. B. PLATT. 1964. Granite out- than ants, as primarypollinators of the species crop communities of the Piedmont Plateau in Georgia. would appear to be the species of small native Ecology 54: 292-305. bees frequently captured on flowers in North CHAPMAN, R. H. 1977. Gene flow and genetic population Carolinaand Georgia. structure in Diamorpha smallii. Ph.D. thesis, Uni- Janzen (1977) has suggested that ants are versity of Georgia, Athens. CLAUSEN, R. T. 1975. Sedum of North America north effectively excluded fromthe nectarof tropical of the Mexican Plateau. Cornell University Press, flowers by "chemicals that are powerfully re- Ithaca, New York. pugnant,indigestible, or toxic to ants." Baker COLE, A. C. 1940. A guide to the ants of the Great Smoky and Baker (1975, 1978) also have commented Mountains National Park, Tennessee. Amer. Midl. on the productionof alkaloids, phenolics, and Nat. 24: 1-88. non-proteinamino acids as possible deterrents COTTER, D. H., AND R. B. PLATT. 1959. Studies on the ecological life history of Portulaca smallii. Ecology to nectar-robbingby ants. The exact circum- 40: 651-668. stances selecting for such chemical defenses CREIGHTON, W. S. 1950. The ants of North America. have been explored by Feinsingerand Swarm Bull. Mus. Comp. Zool. Harv. Coll. 104: 1-585. (1978), who also attemptedto investigate Jan- DENNIS, C. A. 1938. The distribution of ant species in zen's hypothesis experimentally. They con- Tennessee with reference to ecological factors. Ann. cluded that plants pollinated by other hyme- Entomol. Soc. Amer. 31: 267-308. FAEGRI, K., AND L. VAN DER PIJL. 1971. The principles nopterans, by butterflies, or by birds, are of pollination ecology. Pergamon Press, Toronto. unlikely to secrete ant-repellent chemicals, FEINSINGER, P., AND L. A. SWARM. 1978. How common although two species of plants they tested are ant-repellent nectars? Biotropica 10: 238-239. showed ant-repellentqualities. Nectar-robbing GILBERT, L. E., AND P. H. RAVEN. 1975. Coevolution

This content downloaded on Tue, 1 Jan 2013 09:57:50 AM All use subject to JSTOR Terms and Conditions October, 1981] WYATT-ANT-POLLINATION OF DIAMORPHA 1217

of animals and plants. University of Texas Press, Diamorpha smallii and Sedum pusillum. J. Elisha Austin. Mitchell Sci. Soc. 87: 103-114. HAGERUP,0. 1943. Myre-Best0vning. Bot. Tidsskr. 46: SMITH, M. R. 1934. A list of the ants of South Carolina. 116-123. J. N.Y. Entomol. Soc. 42: 353-361. HEINRICH, B., AND P. H. RAVEN. 1972. Energetics and . 1947. A generic and subgeneric synopsis of the pollination ecology. Science 176: 597-602. United States ants, based on the workers (Hymenop- HICKMAN,J. C. 1974. Pollination by ants: a low-energy tera: Formicidae). Amer. Midl. Nat. 37: 521-647. system. Science 184: 1290-1292. . 1951. Formicidae. In C. F. W. Musebeck, K. V. JANZEN,D. H. 1977. Why don't ants visit flowers? Bio- Krombein, and H. K. Townes [eds.], Hymenoptera tropica 9: 252. of North America north of Mexico-synoptic catalog, KEEVER, C. 1957. Establishment of Grimmia laevigata p. 778-875. U.S.D.A. Monogr. 2: 1-1420. on bare granite. Ecology 38: 422-429. SPONGBERG, S. A. 1978. The genera of Crassulaceae in KERNER,A. 1878. Flowers and their unbidden guests. the southeastern United States. J. Arnold Arbor. Kegan Paul, London. Harv. Univ. 59: 197-248. MCCORMICK,J. F., AND R. B. PLATT. 1964. Ecotypic WARE, S. 1972. Growth and dormancy in Talinum rhi- differentiation in Diamorpha cymosa. Bot. Gaz. 125: zomes. Ecology 53: 1196-1199. 271-279. WHEELER, W. M. 1904. The ants of North Carolina. MCVAUGH,R. 1943. The vegetation of the granite flat- Amer. Mus. Nat. Hist. 20: 299-306. rocks of the southeastern United States. Ecol. Mono- 1913. A revision of the ants of the genus Formica gr. 13: 119-166. (Linne) Mayr. Bull. Mus. Comp. Zool., Harv. Coll. MULLER, H. 1883. The fertilization of flowers. Mac- 53: 379-565. Millan, London. WIGGS, D. N., AND R. B. PLATT. 1962. Ecology of Dia- MURDY, W. H. 1968. Plant speciation associated with morpha cymosa. Ecology 43: 654-670. granite outcrop communities of the southeastern Pied- WILBUR, R. L. 1964. Notes on the genus Diamorpha mont. Rhodora 70: 394-407. (Crassulaceae). Rhodora 66: 87-92. PALMER, P. G. 1970. The vegetation of Overton Rock . 1977. Nomenclatural notes on the genus Dia- Outcrop, Franklin County, North Carolina. J. Elisha morpha (Crassulaceae). Taxon 26: 137-139. Mitchell Sci. Soc. 86: 80-87. WYATT, R. 1977. Arenaria alabamensis: a new combi- PROCTOR, M., AND P. YEO. 1972. Pollination of flowers. nation for a granite outcrop endemic from North Car- Taplinger, New York. olina and Alabama. Bull. Torrey Bot. Club 104: 243- RADFORD, A. E., H. E. AHLES, AND C. R. BELL. 1964. 244. Manual of the vascular flora of the Carolinas. Uni- . 1980. The impact of nectar-robbing ants on the versity of North Carolina Press, Chapel Hill. pollination system of Asclepias curassavica. Bull. SHARITZ,R., AND J. F. MCCORMICK.1973. Population Torrey Bot. Club 107: 24-28. dynamics of two competing annual plant species. , AND N. FOWLER. 1977. The vascular flora and Ecology 54: 723-740. vegetation of the North Carolina granite outcrops. SHERWIN, P. A., AND R. L. WILBUR. 1971. The contri- Bull. Torrey Bot. Club 104: 245-253. butions of floral anatomy to the generic placement of

This content downloaded on Tue, 1 Jan 2013 09:57:50 AM All use subject to JSTOR Terms and Conditions