On the Cover

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Mapl es, ge nus Acer, are abunda nt trees or shru bs of northern tempera te cl ima tes, with 13 nati ve species of we ll more than 100 spec ies in No rth A me rica .' > Box elde r (ca lled M anitob a mapl e in Ca nada) , Acer negundo , is th e most wide ly dis tributed nati ve mapl e , prol ific e no ug h to be described as " weedy and ubi quitou s." ? Di stribution of bo x elder is from the Eas tern seaboa rd to the G rea t Plain s, ex te nd ing do wn to the G ulf coast and central T exas. It is sca tte red throu gh ou t the Rocky M ountain s bet ween 3,5 00 a nd 8,500 fee t.v' In Ca lifo rn ia, it is most prevalent in the Bay area and northern Ce ntra l Va lley. In Ca na da, box e lde r is fo und fro m Ontario to wes te rn A lbe rta, and co m mo nly in M anit ob a and Saskatc hewan ." It is hardy in these pro vinces, and has bee n planted the re for sha de and she lte rbe lts . Box el der is fo und in mo ist alluv ia l so ils, in floodplain swamps and moi st bott omlands, co mmo nly associated with co tto nwood and w illow. Box e lde rs are sha de -into lera nt, fas t growi ng, but sho rt-live d, w ith a lifesp an of 60 years'. St umps spro ut vigorous ly after c utting o r fire .l-' It is susce ptible to heart ro t and attrac ts box e lder bu gs (Leptocoris trivittatus). In areas w itho ut sugar mapl e, the sweetish box e lde r sa p has been used to produce a weak mapl e syrup.' Bo x e lders are small to medium trees, usually 50 fee t, rarely 65 . Th e trun k divides near the gro und, with spread ing, croo ked limbs that branch freq uently and irregul arly , with a broad , uneven crow n. Bark o n yo ung trunk s is smoo th and light gray -brown, w ith o lder trunks darker, furrowed w ith narrow ridges. Box e lder is the only native maple with co mpound leaves, resembling ash. Th ey are pinn ately co mpound with three to nine leaflets, 2 to 4 inches in length , coarsely too thed. Leaves are light gree n. gray -green beneath , and ye llow in the fall. T he seed porti on of the samara is narrow and wrinkled; the ang le of the wings acute, < 45°.' Altho ugh many maples are amphiphi lous, box elder is entire ly windpollinated . Pollen flowers and seed flowers are on separa te trees. Anthesis occ urs between April and May; in eas tern Canada in ear ly May, and in western provinces, late M ay.2.5 Box elder is the most prolific airbo rne pollen produ cer amo ng maples, acco unting for the greatest degree of Ace r-induced hay fever.' Ast hma has not been reported. Previou sly co nsi de red members of the two-gen era fa mily, Aceraceae, mapl es and box e lder ha ve recentl y been placed in Sap indaceae, base d on morphology and rbcL chloroplast gene seq uencing ," Cross-reac tivity data on maples are sca nt. Prausnitz-K ilstner neut ralization data showed crossinhibition between maple and Fagales members, but radioallergosorbent test inhibitio n using box e lder solid ph ase showed poor inhibition with most trees tested, and none with hazel , birch , or alder.P Additio nally, there is poor skin test corre lation betw ee n box elde r and red maple." Th is lends support to an older class ification of box elder as a separa te ge nus, Negundo aceroides.

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Box Elder Box e lde r grai ns are tricolpate, rarel y d icolpate. Grains are slightly prol ate, 24 to 30 p.m X 27 to 34 urn, Furrow s are approxima tely 26 p.m lon g, stre tching fro m pol e to pole . Exi ne is appro ximatel y 1.5 p.m thi ck with a sem itectate struc ture. S urface texturing is undul ating rugul ate , not striate as in other members of th e ge nus, with sma ll pit s th at are mor e prominent a long the furro ws ' edge .' RICHARD W . W EB ER, MD Cover. Table of Contents, and Insert photographs courtesy of Richard W. Weber. MD and Greer Laboratories

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REFERENCES I. Farrar JL. Tree s o f the No rthern United Sta tes and Ca nada . Ames, IA: Iowa State Universi ty Press. 1995:132-1 55. 2. Barnes BV. Wagner WH oMich igan T rees, An n Arbo r, MI : University of Michigan Press , 1981: 187- 205. 3. Lewi s WH oVinay P, Zenge r VE. Airborne and Allergenic Pollen of North Ame rica. Baltimore, MD: Johns Hopkins University Press. 1983:12-5. 182. 4 . Elmo re FH. Sh rubs and T rees o f the Sout hwes t Uplands. T ucson, AZ: So uthwes t Parks and Mon umen ts Associatio n. 1976:46. 5. Bassett 11. Cromp ton CWoPannalee JA. An Atlas of Airborne Pollen Grains and Com mon Fungus Spore s of Canada. Hull. Quebec: Printing and Publishing. Supply and Services Canada, 1978:101-102. 6. Judd WS. Cam pbell CS. Kellogg EA. Stevens PF. Plant Systematics: A Phylogenetic Approac h. Sunderland. MA: Sina uer Associa tes. 1999:333- 342. 7. Rackem an n FM. Wagner He. The desensitization skin sites passively sensitized with serum of patients with hay fever. Crosse d reactions of different pollens. The variations in the recipient. J AIlergy 1936;7:319 - 332. 8. Bernstei n IL. Perera M. Ga llagher J. et al. In vitro crossallergenici ty of maj or aeroa llerge nic pollens by the radioa llergosorbent techn ique. J Allergy Clin Immu nol 1976;57: 141-152. 9. We ber RW. Cros s-reactivity of plant and animal allergens. Clin Rev Allergy Imm unol 200 1;2 1:153-202.

Requests f or reprints should be addr essed to: Richard W. Weber, MD Department of Medicine National Jewish Medical. Research Center 1400 Jackson Street Denver. CO 80206 I,

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