872 AMERICAN JOURNAL OF BOTANY [Vol. 50 LITERATURE CITED ---, AND ---. 1928. Kann die Transpiration aus einen multiperforaten Septum die einer gleich grossen BROWN, H. T., AND F. ESCOMBE. 1900. Static diffusion Wasserflache erreichen? Planta 5: 616-621. of gases and liquids in relation to the assimilation of carbon and translocation in plants. Phil. Trans. Roy. =---, AND ---. 1929. Weitere Untersuchungen Soc. London 193B: 223-291. tiber die Verdunstung aus multiperforate Folien mit ECKERSON, S. F. 1908. The number and size of the kleinsten Poren. Planta 9: 246-269. stomata. Bot. Gaz. 46: 221-224. STEFAN, J. 1881. Ueber die Verdampfung aus einen HUBER, B. 1930. Untersuchungen tiber die Gesetze der kreisformig oder elliptish begrenzten Becken. Sitz. Porenverdunstung. Zeitschr. Bot. 23: 839-891. Akad. Wiss. Wien II, 83: 943-954. JEFFREYS, H. 1918. Some problems of evaporation. THOMAS, N., AND A. FURGUSON. 1917. On evaporation Phil. Mag. 35: 270-280. from a circular water surface. Phil. Mag. 34: 308-321. MITCHELL, J. W. 1936. Effect of atmospheric humidity on rate of carbon dioxide fixation by plants. Bot. Gaz. VERDUIN, J. 1949. Diffusion through multiperforate 98: 87-104. septa, p. 95-112. In J. Franck and W. E. Loomis, SAYRE, J. D. 1926. Physiology of stomata of Rumex [ed.], Photosynthesis in plants. Iowa State College patientia. Ohio Jour. Sci. 26: 233-266. Press, Ames, Iowa. SIERP, H., AND A. SEYBOLD. 1927. Untersuchungen zur ---, AND W. E. LOOMIS. 1944. Absorption of carbon Physik der Transpiration. Planta 3: 115-168. dioxide by maize. Plant Physiol. 19: 278-293. THE MORPHOLOGY OF THE INFLORESCENCE OF RAGWEEDS (AMBROSIA-FRANSERIA: COMPOSITAE) 1,2 WILLARD W. PAYNE Department of Botany, The University of Michigan, Ann Arbor, Michigan ABSTRACT PAYNE, WILLARD W. (D. Michigan, Ann Arbor.) The morphology of the inflorescence of rag­ weeds (Ambrosia-Franseria: Compositae). Amer. Jour. Bot. 50(9): 872-880. Illus. 1963.-The ragweeds possess an inflorescence which is highly modified in relation to anemophily. Several trends from primitive to specialized character expressions may be seen in the morphology of the unisexual capitula and florets. An evolutionary scheme is presented whereby the catkinlike, acropetally maturing, staminate spike and the sessile, centrifugally developing, fruiting involucre cluster are derived from basic inflorescence types within the Compositae. Morphological evidence suggests that: (1) the taxa Ambrosia and Franseria should be combined under the older generic name Ambrosia; (2) the ragweeds and their relatives as a group appear to occupy a position intermediate between the Heliantheae and Anthemideae, as currently delimited within the Compositae. THE RAGWEED species, currently divided be­ is to illustrate the salient features of the ragweed tween the genera Ambrosia L. and Franseria inflorescence, both for its intrinsic interest as an Cav., possess one of the most highly modified unusual member of one of our most advanced inflorescence types found in the Compositae. phanerogamic families, and to serve as ground­ This modification has come about primarily work for taxonomic treatments to be presented in through specialization in relation to wind pollina­ the future. nation (anemophily). The unusual floral mor­ 110RPHOLOGY OF THE FLORAL STRUCTURES-­ phology of the ragweeds, similarly expressed in Ragweeds possess floral heads or capitula of 2 varying degrees by related genera (viz. Iva, kinds: (1) staminate heads which characteristi­ Dicoria, Euphrosyne, H ymenoclea, Xanthium), has cally bear only pollen-producing florets; and (2) led to taxonomic confusion at all levels from the pistillate heads which bear 1 or a few seed­ subspecies to the tribe. The purpose of this paper producing florets. Staminate heads are arranged in acropetally maturing, racemelike or spikelike 1 Received for publication February 7, 1963. 2 Publication no. 35 on Atmospheric Pollution by Aero­ clusters at the tips of the stems and branches. allergens, Botanical Phase, under research grant no. Pistillate heads occur in basipetally (centrifugally) E-4347 from the National Institute of Allergy and In­ maturing clusters in the axils of leaves immedi­ fectious Diseases, D. S. Public Health Service. I wish to ately below the staminate spikes (Fig. 1A). express my appreciation to Billie L. Turner. (D. of Texas) and Warren H. Wagner, Jr. (D. of Michigan) for advice The staminate head-The staminate heads of on the content of this paper. most ragweeds are stalked and are not subtended B ~ E D Fig. 1. Floral morphology in Ambrosia . A. Habit sketch of A. irifida; pointers indicate staminate and pistillate portions of th e inflorescence, XO.2S. B. Sta minate head of Franseria camp horaia showing central insertion of th e stalk and pro­ noun ced involucrallobes (phyllary tip s), X S. C. Staminate head of Ambrosia br1jantii X S. D. Sta minate head of A. psilostachua showing eccentric attac hment of stalk and reduction of involucral lobes, X S. E. Staminate head of A . biden­ tata showing loss of stalk and extreme distal prolongati on of involucre XS. F . Dissected staminate floret of A. artemisii­ folia (stamens have become separated in dissection), X IS. G. Fruiting involucre of A. acanlhicarpa showing many scat­ tered, flattened spines, X 5. H. Fruiting involucre of Fran seria ambrosioides with hooked spines, X 5. 1. Fruiting involucre of Ambrosia trifida with reduced spines in a single whorl below th e beak, XS. J . Fruiting involu cre of A . psiloslachsja with vestigial spines, XS. 874 AMERICAN JOURNAL OF BOTANY [Vol. 50 \ A o G Fig. 2. A. Lanceolate pale of Franseria chenopodijolia, X ca. 25. B. Filiform pale of Ambrosia bidentata-X ca. 50. C. Attenuated anther tips of A. psilostachya, X ca. 50. D. Equatorial view of acetolyzed pollen grain of A. bidentata showing 3 pores and intercolpoidal air spaces developed between the endexine and ectexine, X ca. 1050. E. Pistillodium from October, 1963] PAYNE-INFLORESCENCE OF RAGWEEDS 875 by bracts. Orientation of the involucre and florets composed of short, compactly arranged baccula tends to be outward or downward so that pollen (rods, muri)." . is discharged and falls directly into the air. The The pistil (pistillodium) of the staminate floret phyllaries arise in a single whorl and are laterally is highly reduced and modified. Its ovary and connate, so that the involucre is campanulate or stigmatic lobes are essentially lacking, and the saucer-shaped. The tips of the phyllaries may be columnar style is surmounted by a flat (peltate), prominent as marginal teeth or lobes (Fig. 1B), brushlike cluster of hairs (Fig. 2E). After initial or the margin of the involucral cup may be pollen discharge, the pistillodiurn pushes through shallowly lobed or nearly without lobes (Fig. IC, the anther ring sweeping residual pollen from the D). Attachment of the involucral stalk to the floret. involucre varies from central to more or less The pistillate head-Pistillate heads range from eccentric. If the latter, the widest portion of the 2 to 25 mm in length. The involucre is typically involucre is distal in relation to the main stem. In sessile and, with the exception of the central one species, Ambrosia bidentata, the staminate (terminal) head of a cluster, each is subtended by a involucre is sessile or subsessile and the distal more or less leafy bract. The bases of the phyllaries edge is prolonged into a pronounced tooth as long are coalescent to form an obovate or vase-shaped, as or longer than the main body of the involucre indehiscent, usually hard conceptacle within which (Fig. IE). the florets and fruits mature. The tips of the The receptacle of the staminate head is rela­ phyllaries .of the pistillate involucres of most tively small (usually less than 2 mm in diameter) species are evident as spines or tubercles (Fig. and bears from 10 to 150 or more florets which IG-.J). mature centripetally in a gradual succession. Considerable variation in spine development Each floret is subtended by a bract (pale) which and morphology is found among the different may be lanceolate (Fig. 2A) and longer than the ragweed species. In some the spines are quite floret or reduced and hairlike (Fig. 2B). phyllary-like, being flattened, lanceolate, and Staminate florets (Fig. IF) vary from approx­ scattered in several whorls over the surface of the imately 2 to 4 mm in length. There is no pappus. involucre (Fig. IG). In many species the spines are The corolla is campanulate, 5-lobed (4-lobed terete and may be hooked (Fig. IH) or straight. forms occur), regular and possesses only com­ In the most extreme expressions, the spines are missural veins. It usually bears, in addition to blunt or vestigial and are arranged more or less in non-glandular trichomes, small biseriate glands a single whorl near the apex of the fruiting (colletel's) that are scattered over the outer sur­ involucre (Fig. II, J). face. Pistillate florets are borne in small numbers The stamens are weakly connate around the (2-7) or singly within each pistillate involucre. style and do not bear basal tails or appendages. There are no pales. In those species which bear 2 Their claws fit together to close the anther ring or more florets per involucre, the florets are set until the time of anthesis and pollen discharge apart by woody walls which divide the interior of (Bianchi, Schwemmin, and Wagner, 1959) and the involucre into compartments. The florets are may be very attenuated (Fig. 2C). The filaments devoid of calyx, corolla, and androecium and are inserted alternately with the corolla lobes and are usually free. 3 These observations on pollen and pollen structure are Ragweed pollen is tricolpate (tetracolpate based upon light-microscope studies of acetolyzed pollen grains are not uncommon in certain species) of species of ragweeds and ragweed relatives as follows: with reduced furrows that commonly extend little Iva- (sensu lato) 9 species (Jackson [1960] lists 15 species); Diet-ria.