Review articleSupported by a grant from Zeneca Pharmaceuticals

Pollen identificationRichard W Weber, MD*

Learning Objectives: The purpose of this review is to introduce the reader to asystematic approach to categorizing pollen types. This will enable the reader torecognize pollen characteristics of the most common botanical sources and deter-mine the relevant contributors to the local aeroallergen burden.Data Sources: Allergenic plant and pollen atlases for pollen characteristics,

allergy texts for procedures and overviews, and relevant reviews from the Englishmedical literature (1985 to present).Results: By applying a visual gestalt utilizing grain number, size, shape, surface

structures, and internal detail, one is able to identify pollen source to the appropriatebotanical taxonomic level. Identification may be possible only to the family ororder, but most frequently to the genus, and occasionally to the species.Conclusions: Outdoor sampling allows the identification of aeroallergen burden

in a locale. In conjunction with field work, the relevant sources can be identified aswell as time of pollination. This allows the physician to correlate symptoms withexposure and relevant sensitization.

Ann Allergy Asthma Immunol 1998;80:1417.

INTRODUCTIONThe purpose of this review is to pro-vide a basis for identification of themajority of aeroallergenic pollens.Through the consideration of severalvariables discussed below, the ob-server can develop a system of patternrecognition, or gestalt, to assist in char-acterizing the airborne burden in a spe-cific locale. This should be combinedwith a familiarity with what plants areactually growing in the area of con-cern. This may be best achieved bycontacting a botanist who has knowl-edge of the local flora. Such a personmay be working for a community orstate government, or perhaps a univer-sity extension. Additionally, there are anumber of texts that catalogue regionalplants, and the interested party will

find these helpful resources.13 Fieldtrips at varying times of the year willfamiliarize one with the times of an-thesis for the local plants. Knowing thelocal flora will assist in identificationof the particles found on the sampler;this in turn will allow interpretation ofpatients symptom chronology and therelevance of in vivo or in vitro sensi-tivity.This review will use numerous de-

scriptive phrases that are not commonusage. These terms are definedthroughout the text. For ease of refer-ral, these phrases are placed in boldface where they are defined.

BACKGROUNDHigher plants use two methods of pol-len dispersal: wind or vector. While avariety of animals may transport pol-len, insect vectors are by far the mostprevalent. Plant pollinated through theagency of an insect carrier are calledentomophilous. Only about ten per-cent of higher plants utilize wind pol-lination, these are called anemophi-lous, and are the principle cause of

pollinosis. Plants that typically areboth insect-pollinated and wind-polli-nated are amphiphilous. Some pri-marily entomophilous plants, such aslinden (Tilia), produce a large enoughpollen load that appreciable surplusamounts become airborne.

SAMPLE PREPARATIONPrinciples of aeroallergen samplinghave been reviewed previously byBurge.4 The reader is referred to thatarticle for discussion of techniques andtabulation. Advantages of various sam-plers will not be discussed here otherthan to state than older gravimetrictechniques such as the Durham sam-pler are no longer adequate, and volu-metric sampling is strongly recom-mended.Once the sample is obtained, it is

necessary to prepare it for microscopicexamination. Pollens may be stainedwith numerous agents, but the standardpreparation is with Caberlas solution,which utilizes basic fuchsin. This stainpermanently stains the pollen outerwall (exine), while not staining thegreat majority of mold spores; it im-parts a pink to red color to the pollengrains, which persists in permanentmounts. The formulation of Caberlassolution is as follows5:

glycerin5.0cc

95% ethyl alcohol10.0cc

distilled water15.0cc

saturated basic fuchsin2 drops (or 50mg powder)

If the solution is staining too in-tensely, it may be diluted with the dye-free solution.

* From Department of Medicine, Allergy/Clinical Immunology Division, NationalJewish Medical and Research Center, Denver,Colorado.Received for publication August 11, 1997.Accepted for publication in revised form

December 9, 1997.

VOLUME 80, FEBRUARY, 1998 141

STRUCTURE OF POLLENGRAINSThe pollen wall consists of three lay-ers, the outer two comprising togetherthe exine, and the inner layer the in-tine. The exine is resistant to degrada-tion, and withstands acid-heat expo-sure (a process called acetolysis). Theintine is cellulose, and is susceptible tosuch treatment. Beneath the intine liesthe cytoplasm of the grain, or the pro-toplast, enveloped by the cell mem-brane, the plasmalemma (Fig 1).1 Theexine has two layers: the innermostfeatureless, but impervious, nexine,and the outer sexine, which is pene-trated by numerous micropores, aswell as larger apertures: pores and fur-

rows. These apertures are areas ofthinned exine, and act as points ofegress for the pollen tube during suc-cessful pollination. The apertures andmicropores probably also act as ave-nues for the elution of allergenic pro-teins, which may have enzymatic andsignal recognition functions in instigat-ing the fertilization process. The sex-ine may have separate sublayers: anouter roof, or tectum, a foot layerabutting on the nexine, and a centralsegment comprised of columns, orbaculae, separating the two. Addition-ally, the outer surface of the sexinemay have a variety of projections andsurface markings which are discussedbelow.

MORPHOLOGY IN POLLENGRAIN IDENTIFICATIONNumber of GrainsThe majority of pollens captured onsamplers are single grains, or monads.Representative pollen types are shownin Figures 2 and 3. Some, however, aremultiple, such as the 16-grain mimosa(Acacia) polyad, or the 4-grain tetradof broadleaf cattail, Typha latifolia(Fig 2C). The latter is a rhomboidtetrad, while wood rush, Luzula, andmembers of the heath family (Erica)have a tetrahedral configuration. Occa-sionally, pollens will clump together,giving the appearance of a polyad, butwith a little pressure on the coverslip,these will usually separate easily.SizePollen grains vary greatly in size, from2 to 5m to about 250m, althoughmost wind-pollinated grains are be-tween 20 to 60m. Nettle, Urtica, andthe mulberries, Morus and Broussone-tia, all produce pollens in the 10 to15m range. While corn, Zea, andsome conifers, Abies and Pseudotsuga,may be over 100m. Certain ento-mophilous pollens such as pumpkin,Cucurbita, may be as large as 250m.ShapePollen grains are generally elliptical orspherical, but other distinct shapes ex-ist. Pine, fir, and spruce pollens (Pinus,Abies, and Picea, respectively) havetwo air bladders, or vesicles, one eitherside of the grain. This gives the pollengrain a Mickey Mouse cap appear-ance when viewed in the equatorialplane (Fig 2F). These bladders may beeither clear, taking on the fuchsin stain,or be darkly pigmented. Football-shaped or triangular are not uncom-mon, or even wedge-shaped, like a ker-nel of corn (sedge and bulrush, Carexand Scirpus, Fig 2B). Pores in the grainmay act as points on the surface wherethe exine angles, giving the pollen atriangular, square, or polygonal shape,depending on the numbers of pores(ash, Fraxinus, Fig 2E). An exceptionis linden, which has three pores occu-pying the flat sides of a somewhat tri-angular shaped grain, rather than the

Figure 1. Composite cross-sectional detail of pollen grain wall (not all features found on individualgrains). Surface projections not drawn to scale for ease of illustration. See text for definitions.

Figure 2. Photomicrographs showing representative shapes and clusters of pollen grains (not all samescale). A. spherical monoporate monad (grass, Graminae); B. wedge shape (bulrush, Scirpus); C.rhomboid tetrad (broadleaf cattail, Typha latifolia); D. split exine and extruded intine and stellateprotoplast (cedar, Juniperus); E. square shape (ash, Fraxinus); and F. grain with two air bladders (pine,Pinus).

142 ANNALS OF ALLERGY, ASTHMA, & IMMUNOLOGY

corners. In the case of sages (Artemi-sia), the exine invaginates somewhatat the pores, bulging between, givingthe grain a distinctive scallopedappearance.The degree of elongation of ellipti-

cal pollen grains can be used in iden-tification. Kapp suggested using theratio of polar to equatorial diameters,the P/E index.6 The pole of a grainis defined as either the location of asingle pore or midpoint of a singlefurrow, or the end where furrowsconverge (tricolpate or tricolporategrains). The following descriptivephrases are used depending on the P/Eindex:2.0perprolate (very elongated)

1.32.0prolate (slightly elongated)

0.751.3subspheroidal

0.500.75oblate (slightly flattened)

0.5peroblate (very flattened)

Surface StructuresThe majority of pollen grains can beidentified by surface structures in the

sexine, the most distinctive being ap-ertures: pores and furrows (Fig 4). Ad-ditionally, projections off the grainsurface may be characteristic, as maybe ridge patterns on the pollen surface.These structures are the primary meansof identification to the taxonomic levelof genus, and can sometimes delineatespecies.Pores. Pollens with pores are called

porate. Pores are primarily circular,but sometimes elliptical, and may besingle or multiple. The appropriateprefix specifies whether there is a sin-gle pore, two, three, or numerous pores(mono-, di-, tri-, or periporate, respec-tively). Members of the grass familyinvariably aremonoporate, as are sev-eral grass allies (Fig 2A). Triporategrains are perhaps the next most com-mon. A few species have two pores,such as the mulberries and wood nettle(Parietaria), and occasionally aberrantgrains of species normally having 3pores (nettle and birch, Urtica and Bet-ula) will only possess two. Weeds ofthe closely related chenopod and ama-ranth families have multiple pores,generally between 20 and 80 per grain.Under the microscope, these pollens

look like golf balls (Fig 3C). In dis-tinction, plantains (Plantago) havefewer pores, usually 6 to 11. Sweetgum, Liquidambar, has 12 to 20 pores,which are bulging with dark granules.Grains with 4 pores oriented alongthe grains equator (crown-like) arecalled stephanoporate (Fig 3). Forsome periporate pollens such as wal-nut, Juglans, the pores are entirely onthe equator and one hemisphere.There are several modifications of

the exine around pores which may behelpful in classification (Fig 5). Thepore sometimes possesses a cap orplug, the operculum. An aspis is athickening of the exine around a pore,like a shield. An annulus, or ring,around the pore may either be a thick-ening, or a thinning of the exine. Athickening of the intine under a pore iscalled an oncus. A band may arch be-tween pores, and is called an arcus;although it may appear to be within theinner portion of the grain, it is thick-ened sexine, forming a ridge along thesurface of the grain. Birch is a triporate

Figure 4. Types of pollen apertures (struc-tures depicted in dotted lines are situated on farside of pollen grain). A. inaperturate; B. mono-colpate; C. tricolpate, equatorial plane; D. tricol-pate, polar view; E. tricolporate; F. periporate;G. monporate; H. triporate, equatorial plane; I.triporate, polar view; and J. stephanoporate.

Figure 3. Photomicrographs showing representative pollen grain surface features (not all same scale).A. three-furrowed, tricolpate (maple, Acer); B. spined, echinate, tricolporate (sunflower, Helianthus); C.periporate chenopod-amaranth; and D. triporate grain with distinctive aperture features of annulus, aspis,and oncus (birch, Betula).

VOLUME 80, FEBRUARY, 1998 143

pollen with a very distinctive annulus,aspis, and oncus (Fig 3D). Alder, Al-nus, which is pentaporate, likewise hasdistinctive aspides and very prominentarci. While some species of birch mayhave arci, these are much less apparentthan with alder grains.7,8Furrows (colpi). These are slits or

boat-shaped areas of thin exine (Fig 4).Grains with furrows are called colpate.The furrows may be quite long,stretching from pole to pole, or bequite short and unapparent. The sameprefixes are used as with pores to des-ignate number and orientation. Pollenswhich possess both furrows and poresare called colporate. The pores in suchcases are located within the furrows,which occasionally may be quite short,barely extending beyond the pore, asseen with the ragweeds, Ambrosia spp.Maples and oaks (Acer and Quercus)are colpate but may show bulging fur-rows which appear colporate in polarview (Fig 3A).Sculpturing. Surface texturing into

ridges or depressions may occasionallyidentify one species from another, al-though such detail is frequently diffi-cult to discern under normal conditionsof light microscopy. A pollen with asmooth surface, such as with thegrasses, is called psilate. Patterns maybe reticulate (net-like), striate (rough-ly parallel ridges), or rugulate (irreg-ular pattern). Most maples have adistinctly striated surface; boxelder,however, has a more irregular pattern.Cottonwoods, Populus, have a granu-lar, irregular surface texture.The roof of the exine may have

prominent projections of variousshapes: baculate, or rod-like; clavate,club-like or tennis-racket shaped; ech-inate, spiny; gemmate, door knobshaped; scabrate, rough or flecked;and verrucate, warty or bumpy (Fig1).6 These projections may be very dis-tinctive, and readily apparent, or maybe very subtle and hard to see, requir-ing careful observation under high res-olution with oil immersion. Manycomposite weeds (including the rag-weeds, Ambrosia) have obvious spines(Fig 3B). Those of the ragweeds areshort and broad-based, while those of

primarily entomophilous species suchas goldenrod and sunflower (Solidagoand Helianthus, respectively) havelonger spines. Sages are baculate, butthe rod-like projections are less easy toappreciate. Bald cypress (Taxodium)and redwood (Sequoia) pollens resem-ble those of the cedars and junipers(see below) with the exception of asingle irregular projection, an exit pa-pilla.Interior StructuresWhile the protoplast of many pollens isnondescript, internal characteristics ofcertain pollens are helpful for identifi-cation. The presence of a thick intinehelps identify cottonwoods. The intinemay be particularly dense, as seen withCypress family members (cedars andjunipers, Juniperus), with an irregularstellate appearance of the protoplast.Juniperus species, particularly, have athin exine, which frequently splitsopen on the microscope slide, extrud-ing the clear round intine and proto-plast portion of the grain, leaving thestained exine appearing like a Pac-man (Fig 2D). Docks and sorrels,Rumex, have prominent internal starchgranules, giving the interior a pop-corn appearance. Grasses also havestarch granules, but these are muchless apparent.

SUMMARY OF MAJORAIRBORNE POLLEN GROUPCHARACTERISTICSThe following section summarizes pol-len attributes of the major groupings of

aeroallergenic plants. It is not meant tobe exhaustive, but rather highlights theaverage appearance and significantcharacteristics within each group. Pol-len characteristics have been derivedfrom several sources.612 Further bo-tanical taxonomic information on theseplants may be found in several sourc-es.1,13,14

PinaceaeMembers of the pine family (pine,spruce, and fir; Pinus, Picea, andAbies, respectively) generally havetwo air bladders, are inaperturate, andrange in size from 40 to 100m.Larches and Douglas fir (Larix andPseudotsuga) do not have vesicles.Hemlocks (Tsuga spp.) may have twobladders, which are either fully formedor rudimentary, or have none.

CupressaceaeCedars, junipers, and arbor vitae (Ju-niperus and Thuja spp.) are psilate andinaperturate, and smaller than pines,ranging from 20 to 35m. A markedlythickened intine is up to 6.5m.

TaxodiaceaeBald cypress and redwoods (Taxodiumand Sequoia spp.) are up to 36m,with a thickened intine. They charac-teristically have a single exit papilla.

Graminae (Poaceae)Grasses are all very similar in appear-ance: psilate, spheroidal, from 20 to60m. There is much overlap in size,and with the exception of corn (90 to110m), this cannot separate genus or

Figure 5. Composite cross-sectional detail of pore-associated structures (not all features found onindividual grains). See text for definitions.

144 ANNALS OF ALLERGY, ASTHMA, & IMMUNOLOGY

species. Grains are monoporate, with aprominent operculum.

Compositae (Asteraceae)Members are tricolporate, with poresand furrows more or less prominent.Ragweeds (Ambrosia) have short fur-rows with indistinct pores, while sageshave more apparent apertures. Short,broad-based spines on ragweeds andmarshelders (Iva), longer spines ongoldenrod, sunflower, daisy (Solidago,Helianthus, Aster, respectively). Sages(Artemisia) without spines, scalloping.Size is between 15 to 30m.

ChenopodialesMembers of the closely related ama-ranth and chenopod families have verysimilar pollens which are periporate,ranging from 20 to 80 pores per grain.Pore size and number show genus andspecies differences, but the overlap issufficient to make identification belowthe level of botanical order very diffi-cult. Grain size is between 20 and35m.

PlantaginaceaePlantains (Plantago) are periporate, butwith significantly fewer pores than thechenopods or amaranths. Usual porenumber is between 6 to 11. Size isbetween 20 and 30m.

BetulaceaeThe birch family has tri- to pentaporategrains, with distinct pore-associatedfindings such as annuli, aspidae, andarci. Birch (Betula) usually has threepores, while alder has 4 or 5. Size isbetween 20 and 35m. Alder (Alnus)with distinct oncus and arci.

FagaceaeThe beech family has tricolpate or tri-colporate members, ranging from 10 to15m (chestnut, Castanea) to 25 to40m (oak and beech, Quercus andFagus). Shapes are from prolate to tri-angular in polar view.

AceraceaeThe majority of Acer species are tri-colpate, with large furrows, and a stri-ate surface. Boxelder (Acer negundo)is tricolpate with a rugulate texture.Vine maple (Acer circinatum) is tricol-porate. Grains range from 25 to 40min size.

JuglandaceaeWalnuts and butternuts (Juglans) areperiporate, with 10 to 15 pores on theequator and one hemisphere (heteropo-lar). Pores are aspidate with indistinctonci. Hickory and pecans (Carya) areusually triporate with pores evenlyspaced close to the equatorial plane.Large oblate grains ranging from 35to 55m.

OleaceaeGrains of ash and olive (Fraxinus andOlea) are tri- to tetracolpate, with shortfurrows. Surfaces are finely orcoarsely reticulate, respectively. Sizevaries from 20 to 30m.

SalicaceaeCottonwood and aspen (Populus) haveinaperturate grains, while willows(Salix) are tricolpate. Willows have areticulate surface and are between 10and 20m. Populus species have agranular texture, and are larger, be-tween 20 and 35m.

UlmaceaeElms (Ulmus) are stephanoporate withusually five pores, although rangingfrom 4 to 7. Surface is wavy and con-voluted. Grains are oblate with equa-torial diameter 25 to 35m. Hackberry(Celtis) pollens have 3 to 10 pores,with a thick undulating, granular sex-ine, 25 to 40m.

REFERENCES1. Lewis WH, Vinay P, Zenger VE. Air-borne and allergenic pollen of NorthAmerica. Baltimore: Johns HopkinsUniversity Press, 1983.

2. Whitson TD, Burrill LC, Dewey SA, et

al. Weeds of the west. Jackson, WY:University of Wyoming, 1991.

3. United States Department of Agricul-ture Forest Service. Range plant hand-book. Dover Publications, New York,1988.

4. Burge HA. Monitoring for airborne al-lergens. Ann Allergy 1992;69:918.

5. Sheldon JM, Lovell RG, Mathews KP.Standard technic for atmosphericpollen testing by gravity method. In:A manual of clinical allergy.Philadelphia: WB Saunders Co, 1953:3636.

6. Kapp RO. How to know pollen andspores. Dubuque, Iowa: Wm C Brown,1969.

7. Brown GT. Pollen-slide studies.Springfield, Illinois: Charles CThomas, 1949:4753.

8. Wodehouse RP. Pollen grains. NewYork: Hafner, 1935.

9. Ogden EC. Manual for sampling air-borne pollen. New York: Hafner,1974.

10. Bassett IJ, Crompton CW, ParmeleeJA. An atlas of airborne pollen grainsand common fungus spores of Canada.Hull, Quebec: Printing and Publishing,Supply and Services Canada, 1978.

11. Smith EC. Sampling and identifyingallergenic pollens and molds. San An-tonio, TX: Blewstone Press, 1984.

12. Smith EC. Sampling and identifyingallergenic pollens and molds. volumeII. San Antonio, TX: Blewstone Press,1986.

13. Weber RW, Nelson HS. Pollen aller-gens and their interrelationships. ClinRev Allergy 1985;3:291318.

14. Solomon WR, Weber RW, Dolen WK.Common allergenic pollen and fungi.In: Bieeman CW, Pearlman DS, Sha-piro GG, Busse WW, eds. Allergy,asthma, and immunology from infancyto adulthood, 3rd edition. Philadel-phia: WB Saunders Company, 1996:93114.

Requests for reprints should be addressed to:Richard W Weber, MDDepartment of MedicineNational Jewish Medical & Research Center1400 Jackson St, Rm B103ADenver, CO 80206

VOLUME 80, FEBRUARY, 1998 145

CME ExaminationNo. 008-002Questions 120: Weber RW. Ann Allergy Asthma Immunol 1998;80:1406CME Test Questions

1. The pigment used to stain pol-len grains in Caberlas solu-tion isa. toluene blue.b. basic fuchsin.c. phenosafranin.d. eosin.e. hematoxylin.

2. The two outer layers of thepollen wall are called thea. ectomorph and endomorph.b. intine and plasmalemma.c. sexine and nexine.d. baculum and protoplast.e. aspis and oncus.

3. Plants that are both wind- andvector-pollinated are calleda. amphiphilous.b. aerobic.c. entomophilous.d. anemophilous.e. anaerobic.

4. Apertures that are elongatedarea. pores.b. colpi.c. operculae.d. papillae.e. vesicles.

5. Grasses are invariablya. diporate.b. tetracolpate.c. monocolpate.d. monoporate.e. monocolporate.

6. Members of the Coniferalesorder which have pollens withair bladders includea. larch, ginkgo, and Austra-lian pine.

b. juniper, cedar, and hem-lock.

c. yew, bald cypress, andjoint pine (Ephedra).

d. myrtle, heath, and NorfolkIsland pine.

e. pine, spruce, and fir.7. A distinguishing feature ofdocks and sorrel (Rumex spp.)grains isa. exit papilla.

b. thin exine that frequentlysplits and extrudes innerportion of grain.

c. prominent starch granules.d. large single furrow.e. more than 60 pores.

8. Members of the Chenopodia-les order (amaranths and che-nopods) are periporate withwhat range of pore number?a. 4 to 11b. 3 to 15c. 12 to 20d. more than 60e. 20 to 80

9. Stephanoporate refers towhat attribute?a. arrow-headed surface orna-mentation

b. pores found within furrowsc. crown-like string of poresaround the grain equator

d. 20 porese. 4 furrows aligned aroundgrain equator

10. The sexine of the pollen graina. may be composed of a tec-tum, series of columns, andfoot layer.

b. is the origin of the pollentube.

c. is the outer portion of theintine.

d. contains the pollens nu-clear material.

e. is susceptible to acetolysis.11. Pollen grains that have no

pores or furrows are calleda. psilate.b. operculate.c. oblate.d. percolate.e. inaperturate.

12. Plantain pollen may be differ-entiated from amaranth pollenbya. being pericolpate ratherthan periporate.

b. having bulging, granularpores.

c. being triporate.

d. having fewer pores, 6 to 11rather than 20.

e. having pores limited to onehemisphere.

13. Grains being very elongatedarea. called perprolate.b. have a P/E index of 0.5.c. called peroblate.d. called echinate.e. called obstinate.

14. Bald cypress or redwood pol-lens may be distinguishedfrom cedar pollen bya. having two air bladders.b. being monoporate.c. having single exit papillae.d. possessing a single vesicle.e. being tetrads.

15. A triporate grain with promi-nent annulus, aspis, and oncusis characteristic of which ofthe following pollens?a. broad-leafed cattailb. mountain cedarc. aspend. plantaine. birch

16. What is an annulus?a. a shield-like thickening ex-tending away from a pore

b. a ring-like thickening orthinning around a pore

c. a string of pores around theequator of a grain

d. an aperture filled with de-bris

e. arrow-shaped surface orna-mentation

17. A pollen grain with surfacepatterns assuming a net-likeappearance is calleda. reticulate.b. rugulate.c. psilate.d. pronate.e. obfuscate.

18. Walnut pollen may be distin-guished from sweetgum pollenbya. walnut has bulging pores.

146 ANNALS OF ALLERGY, ASTHMA, & IMMUNOLOGY

b. sweetgum has 40 to 60pores compared to 20 forwalnut.

c. walnut is heteropolar withpores in one hemisphere.

d. walnut is tricolporate.e. there are no distinguishingcharacteristics.

19. Surface ornamentation that iswarty or bumpy is calleda. gemmate.

b. scabrate.c. clavate.d. echinate.e. verrucate.

20. Cottonwood and grass pollensmay be distinguished fromeach other by all but which ofthe following criteria?a. In certain locales cotton-wood pollinates earlierthan grass.

b. While both are subspheroi-dal, cottonwood is inaper-ture while grasses are mo-noporate.

c. Cottonwood has a rugulateto granular texture whilegrasses are psilate.

d. Cottonwood is triangularwhile grass is perprolate.

e. Grass has an operculum.

VOLUME 80, FEBRUARY, 1998 147

Instructions for Category I CME Credit

Certification. As an organizationaccredited for continuing medical ed-ucation, the American College ofAllergy, Asthma, & Immunology(ACAAI) certifies that when theCME material is used as directed itmeets the criteria for two hourscredit in Category I of the AmericanCollege of Allergy, Asthma, & Im-munology CME Award and the Phy-sicians Recognition Award of theAmerican Medical Association.Instructions. Category I credit can

be earned by reading the text material,taking the CME examination and re-cording the answers on the perforatedanswer sheet entitled, Continuing

Medical Education, which can befound after the examination.Please record your ACAAI identifi-

cation number and the quiz identifica-tion number in the spaces and scanningtargets provided on the answer sheet.Your ACAAI identification numbercan be found on your ACAAI mem-bership card, nonmembers of the Col-lege will be assigned an ACAAI iden-tification number and this should beleft blank on the answer sheet. Thequiz identification number can befound at the beginning of the CMEexamination.Use a No. 2 or soft lead pencil for

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erase but do so completely in order toprevent computer reading errors. YourACAAI identification number and quizidentification number will be used torecord your credit hours earned on theCME transcript system. No records ofindividual performance will be main-tained.Tear out the perforated answer sheet

and print your name and address in thespaces provided. Return it within onemonth after the Annals is received tothe American College of Allergy,Asthma, & Immunology, 85 West Al-gonquin Rd, Suite 550, ArlingtonHeights, IL 60005. Answers will bepublished in the next issue of the An-nals of Allergy, Asthma, & Immunology.

Answers to CME examinationAnnals of Allergy,Asthma, & Immunology, January 1998 (Identification No008001) Vaswani SL and Creticos PS. Metered doseinhaler: past, present, and future. Ann Allergy Asthma Im-munol 1998;80:1123.1. e2. c3. b4. a5. b6. b

7. e8. a9. e10. b11. b12. c

13. e14. d15. e16. c17. e18. a

19. e20. c21. b22. e23. c24. c

25. b26. c27. e

148 ANNALS OF ALLERGY, ASTHMA, & IMMUNOLOGY

Pollen identificationINTRODUCTIONBACKGROUNDSAMPLE PREPARATIONSTRUCTURE OF POLLENG RAINSMORPHOLOGY IN POLLEN GRAIN IDENTIFICATIONSizeShapeSurface StructuresInterior Structures

SUMMARY OF MAJOR AIRBORNE POLLEN GROUP CHARACTERISTICSPinaceaeCupressaceaeTaxodiaceaeGraminae (Poaceae)Compositae (Asteraceae)ChenopodialesPlantaginaceaeBetulaceaeFagaceaeAceraceaeJuglandaceaeOleaceaeSalicaceaeUlmaceae

REFERENCES