Abortive Spores Research Articles

Apogamous Isoetes coromandelina L.f. (Isoetaceae) with asynaptic meiosis

This cytological study on Isoetes coromandelina L.f (Isoetaceae) from Tirunelveli District, Tamil Nadu, South India shows the presence of twenty two individual chromosomes along with a fragment (n=2n=22+1) during meiosis in megaspore mother cells without the presence of any bivalent. Cytomixis between two megaspores is very common, resulting in the formation of a high percentage of abortive spores. The present material is of an apogamous taxon with asynaptic Meiosis as reported by Abraham and Ninan from Kerala and Karnataka.

Variability of in situ spores in some leptosporangiate ferns from the Triassic in Italy and Austria

Spores or pollen from a single species or even an individual plant or sporangium may vary in morphology and size to a point that equivalent forms found dispersed in the sediment have been described as different species or even genera. In addition, not all these organs were mature at the moment of burial, and therefore they do not always contain completely developed microspores. To understand these variations better, we studied the intraspecific and interspecific morphological variability of in situ spores of leptosporangiate ferns (belonging to the Osmundales, possible Osmundales, and Gleicheniales) from the Triassic of Europe. The material comes from the Anisian Kühwiesenkopf/Monte Prà della Vacca flora of Italy and the Carnian Lunz flora of Austria. Our analysis is aimed at distinguishing the normal range of variability, differentiating developmental stages, and determining the frequency of abnormal spores. Results show significant variation in both size and surface ornamentation of spores from the same sporangia, as well as between sporangia and individuals, which may have various causes. Abortive spores are usually rare but occur with higher frequency in different samples from some specimens, suggesting that they can serve to identify systemic defects in the plants.

Independent allopatric polyploidizations shaped the geographical structure and initial stage of reproductive isolation in an allotetraploid fern, Lepisorus nigripes (Polypodiaceae).

Although polyploidy is pervasive and its evolutionary significance has been recognized, it remains unclear how newly formed polyploid species become established. In particular, the impact of multiple origins on genetic differentiation among populations of a polyploid species and whether lineages of independent origins have different evolutionary potentials remain open questions. We used population genetic and phylogenetic approaches to identify genetic differentiation between lineages with independent origins within an allotetraploid fern, Lepisorus nigripes. A total of 352 individuals from 51 populations were collected throughout the distribution range. To examine the genetic structure, multilocus genotyping, Bayesian population structure analysis, and neighbor-net analysis were carried out using single-copy nuclear genes. Phylogenetic trees were constructed to detect recurrent polyploid origins. Proportions of abortive spores were analysed as the measure of postzygotic reproductive isolation. Two genetically distinct lineages, the East-type and the West-type, were distributed mainly in the eastern and western parts, respectively, of the Japanese archipelago. Phylogenetic analyses indicated independent origins of these types and detected additional independent origins within each type. We also revealed limited genetic recombination between both types, even in their sympatric regions. F1 hybrids between the East- and West-types showed a reduction in fertility. It is likely that the East- and West-types formed independently in the eastern and western parts of Japan, respectively. The limited genetic recombination and reduced fertility of hybrids suggest that the two types are at an incipient stage of speciation. Two polyploid lineages with independent geographic origins could develop reproductive isolation barrier(s).

Character expression, reproductive barriers, and origin of the rare fern hybrid Asplenium ×aran-tohanum (Aspleniaceae)

Hybridization is a ubiquitous force in plant evolution. In ferns, hybrids are often easily recognized by their intermediate morphology and abortive spores and thus provide a good model for studying reproductive isolating barriers between species. Asplenium ×aran-tohanum is a rare fern hybrid, despite wide coexistence of its parental species (A. billotii and A. trichomanes subsp. quadrivalens) in Western Europe. We made a complete characterization of its three known individuals, including macro- and micromorphology, sporogenesis, gametophyte reproduction, and chloroplast DNA inheritance and evolution. The hybrid expressed morphological characters that were mostly intermediate between those of the parents, but some characters were more similar to one parent or the other. Sizes of both guard cells and spores indicate that the hybrid is tetraploid, as are both parents, and one parent (A. billotii) consistently acted as the female. A very small fraction of spores (~ 7%) were viable and the resulting gametophytes could not form sporophytes, either sexually or apogamously, suggesting that effective postzygotic barriers exist between the parents. The lineages of these taxa diverged about 35 million years ago, which may explain the strong reproductive isolation and rarity of the hybrid. Asplenium ×aran-tohanum appears to be an evolutionary dead end, probably formed recurrently at the places where it grows, but incapable of completing its life cycle or producing viable offspring.

A new hybrid of Tectaria (Tectariaceae) from southern China

A new natural hybrid, Tectaria × hongkongensis (Tectariaceae), is described and illustrated from Hong Kong, China. It is a sterile hybrid supported by the abortive spores and cytological data [2n = 120 (3x)]. This hybrid is morphologically somewhat similar to T. zeilanica in the small size of plants, strongly dimorphic leaves, the shape of sterile fronds, and acrostichoid sporangia. Molecular data indicate its maternal parent is probably T. harlandii. Tectaria × hongkongensis is the first hybrid confirmed by morphological and cytological evidence in East Asia.

The Hybrid Origin of <I>Adiantum meishanianum</I> (Pteridaceae): A Rare and Endemic Species in Taiwan

Abstract Adiantum meishanianum is an endemic species distributed only in Meishan village, Kaohsiung, Taiwan. Because its sporangia contain only abortive spores, A. meishanianum has been regarded as having a hybrid origin, presumably with A. caudatum, A. malesianum, and A. philippense as the putative parents. The aim of this study is to confirm the hybrid origin and determine the parental lineages of A. meishanianum by examining cytology, reproductive modes, and using molecular phylogenetics. We found that the sequences of two chloroplast regions, rps16-matK intergenic spacer and the matK gene, are identical between A. meishanianum and A. malesianum, suggesting A. malesianum is the maternal parent. The nuclear phylogeny reconstructed based on the low-copy marker, CRY2 first intron, reveals that A. meishanianum has three types of sequences: one type groups with sequences of sexual diploid individuals of A. philippense and the other two group with sequences of the sexual tetraploid A. malesianum, indicating ...

Abnormal spore morphology and wall ultrastructure in Anemia tomentosa var. anthriscifolia and A. tomentosa var. tomentosa (Anemiaceae)

The spores of Anemia tomentosa var. anthriscifolia and A. tomentosa var. tomentosa were studied focusing the attention on their abnormalities. The study was based on fresh and herbarium material and the spores were examined with light, scanning and transmission electron microscopy. Normal, abnormal and abortive spores were observed in both taxa. The normal spores were trilete, triangular in polar view, and the ornamentation consisted of parallel ridges separated by narrow and smooth grooves. The spores were observed in monads, dyads, triads and tetrads. The abnormal spores were monolete, trilete, tetralete or alete with great variations in size. In fact, some spores were almost double the size of the normal ones. Some differences were also found in the ornamentation of the spores. Aborted and not completely developed spores were also observed in the specimens. The wall ultrastructure of the taxa was studied for the first time. The exospore was two-layered with numerous cavities inside its structure, and the perispore was also two-layered. The results revealed that the sporoderm ultrastructure of both normal and abnormal spores of the taxa analyzed was very similar.

Two types of partial fertility in a diploid population of the fern Thelypteris decursive-pinnata (Thelypteridaceae)

Two types of abnormal sporophytes were observed in a population of diploid Thelypteris decursive-pinnata. Most sporophytes in this population exhibited regular chromosome pairing, resulting in the formation of 30 bivalents in meiosis I; however, they produced abortive spores to various degrees. Some formed large globose spores at low frequencies, most likely to be unreduced diplospores. The other type of abnormal sporophyte underwent synaptic failure to form 60 univalents at meiosis I, but produced fertile spores, mostly large globose ones at low frequencies. The globose spores were considered unreduced diplospores because the gametophytes arising from them produced tetraploid sporophytes by gametophytic selfing. One tetraploid formed only univalents at meiosis I. Allozyme variation was not detected in this population, although neighboring ordinary diploid populations exhibited it to a certain degree. The sympatric occurrence and allozyme uniformity of the two groups suggest that both are offspring of a founder sporophyte, which may have possessed two types of mutated recessive genes responsible for the spore sterility and the synaptic failure in meiosis. Unreduced spores formed by these two types may play an important role in the polyploid speciation of this species.

Sexuality and apogamy in the Cuban Asplenium auritum–monodon complex (Aspleniaceae)

Asplenium auritum Sw. is a sexual fern that produces 64 spores per sporangium, while A. monodon Liebm. is an apogamous species that produces 32 spores. The hybrid between them, A. × lellingerianum Sanchez & Regalado, also shows an apogamous life cycle, with mainly abortive spores. The scope of this work was to study the sexual and apogamous behavior in these taxa. We studied spore germination and gametophyte development of the three species, a process that followed the Adiantum type. Afterward, we observed morphological apogamous characters in A. monodon and A. × lellingerianum. Apogamous sporophytes arose from apical and basal regions of gametophytes, lacking feet and roots in the first instance, but developing other normal sporophytic structures, such as tracheids, stomata, glandular hairs, and scales. Finally, we studied the gametangia production in all three taxa, finding that the scarce production of antheridia in A. monodon is indicative of the Braithwaite apogamous life cycle scheme, and this scheme has probably been inherited by A. × lellingerianum.

×Cyathidaria, a new nothogenus in theCyatheaceae (Pteridophyta)

Caluff, M. G.: ×Cyathidaria, a new nothogenus in the Cyatheaceae (Pteridophyta). — Willdenowia 32: 281–283. 2002. — ISSN 0511-9618.The nothogenus ×Cyathidaria is established to accommodate various natural hybrids between species of Cyathea and Cnemidaria, all characterised by abortive spores and morphologically intermediate between their respective parent species. ×Cyathidaria acunae, the natural hybrid between Cnemidaria horrida and Cyathea aspera is a newly described nothospecies from Cuba. Four new combinations in ×Cyathidaria are made.

Cibotium Xheleniae Hyb. Nov. (C. chamissoi X C. menziesii; Cyatheaceae): A Naturally Occurring Hybrid from Oahu, Hawaii

The hybrid between Cibotium chamissoi and C. menziesii is described from the Ha- waiian island of Oahu. Distinctions between the hybrid and its parents are detailed. The potential existence of other hybrids among the four Hawaiian Cibotium species is discussed. Intensive investigations of Hawaiian pteridophytes over the past decade have revealed many interspecific hybrids, both published (Ranker, 1992; Wag- ner, 1993; Wagner et al., 1995; Palmer, 1997) and unpublished. These are rec- ognized usually by their coexistence with the parental species, morphological intermediacy, mostly sporadic occurrence, and more or less abortive spores. Here, I describe a recently discovered natural hybrid between two of the more common Hawaiian species of tree ferns of the genus Cibotium. The genus Cibotium includes about ten species ranging from Assam through southeastern Asia to Borneo and the Ryukyus, as well as other Pacific Islands, Mexico, and Central America (Kramer, 1990), with four species recognized in Hawaii (Palmer, 1994). The present paper describes a distinctive hybrid be- tween Cibotium chamissoi Kaulf.and C. menziesii Hook., presently known only from the island of Oahu, but probably more widespread on other Hawai- ian islands.

Electrophoretic evidence for the origins of tetraploids and hybrids ofSphenomeris chinensis sensu lato (Lindsaeaceae) in Japan

Hypotheses for the origins of tetraploids and hybrids ofSphenomeris chinensis sensu lato in Japan were tested using electrophoretic analysis of isozymes. Comparison of banding patterns supported the hypothesis that plants having abortive spores had resulted following hybridization between diploidS. chinensis var.chinensis andS. biflora and between diploid var.chinensis andS. gracilis. Results from isozymic analyses also revealed thatS. intermedia probably originated through allotetraploid speciation following hybridization between var.chinensis andS. biflora. There were two electrophoretic variants among tetraploids ofS. chinensis. One variant, restricted to the southern Ryukyu Islands, is probably an allotetraploid of hybrid origin between diploidS. chinensis andS. gracilis. It has been described as a new species,S. yaeyamensis Lin, Kato & K. Iwatsuki. The other variant, morphologically corresponding to var.chinensis, is distributed at least throughout southern Japan and also in northern Thailand. Its origin is uncertain, but one of the parents is probably diploidS. chinensis. Sphenomeris intermedia andS. yaeyamensis occur inland, whereas the parentalS. biflora andS. gracilis are halophilous and rheophllous, respectively.

INCIPIENT POLYPLOID SPECIATION IN THE MAIDENHAIR FERN (ADIANTUM PEDATUM; ADIANTACEAE)?

Despite the widespread occurrence of polyploids in plant taxa and the many advantages attributed to polyploidy, very little is known about the specific processes that lead to the establishment of polyploids in nature. Classical models suggested that polyploids arise following somatic chromosome doubling in hybrids. However, the production of polyploids from unreduced meiotic products has been receiving greater attention. During an enzyme electrophoretic study of a local population of Adiantum pedatum, a mutant producing viable unreduced spores along with abortive spores was discovered. Studies of sporogenesis showed that a synaptic mutation caused the paired chromosomes to disassociate, with mostly univalents remaining at metaphase I. In such aberrant spore mother cells, one of two pathways was followed in the remaining stages of meiosis. Cells attempting both meiotic divisions formed abortive spores. However, in spore mother cells that bypassed meiosis I and formed a restitution nucleus, meiosis II and subsequent stages of sporogenesis proceeded normally. Unreduced diploid spores resulted from this second pathway. When sown on either agar or sterile soil, these diplospores germinated and produced diploid gametophytes. Tetraploid sporophytes were produced by the gametophytes growing on sterile soil. The discovery of diploid sporophytes producing unreduced spores provided the opportunity to characterize the first step of one possible route to polyploid formation. Continued observations of the natural population may provide insights into the earliest stages of natural polyploid formation.

Incipient Polyploid Speciation in the Maidenhair Fern (Adiantum pedatum; Adiantaceae)?

Despite the widespread occurrence of polyploids in plant taxa and the many advantages attributed to polyploidy, very little is known about the specific processes that lead to the establishment of polyploids in nature. Classical models suggested that polyploids arise following somatic chromosome doubling in hybrids. However, the production of polyploids from unreduced meiotic products has been receiving greater attention. During an enzyme electrophoretic study of a local population of Adiantum pedatum, a mutant producing viable unreduced spores along with abortive spores was discovered. Studies of sporogenesis showed that a synaptic mutation caused the paired chromosomes to disassociate, with mostly univalents remaining at metaphase I. In such aberrant spore mother cells, one of two pathways was followed in the remaining stages of meiosis. Cells attempting both meiotic divisions formed abortive spores. However, in spore mother cells that bypassed meiosis I and formed a restitution nucleus, meiosis II and subsequent stages of sporogenesis proceeded normally. Unreduced diploid spores resulted from this second pathway. When sown on either agar or sterile soil, these diplospores germinated and produced diploid gametophytes. Tetraploid sporophytes were produced by the gametophytes growing on sterile soil. The discovery of diploid sporophytes producing unreduced spores provided the opportunity to characterize the first step of one possible route to polyploid formation. Continued observations of the natural population may provide insights into the earliest stages of natural polyploid formation.

The Appalachian Firmoss, a New Species in the Huperzia selago (Lycopodiaceae) Complex in Eastern North America, with a New Combination for the Western Firmoss

The gemmiferous firmosses, containing the type species of Huperzia, Huperzia selago, are distinguished from the other members of Huperzia by concave-sided spores and specialized lateral branches consisting of gemmiphores and detachable gemmae. The species of the group hybridize and the hybrids possess abortive spores with spore fragments and undivided spore mother cells. In the past the systematic treatment of the species in this group has been unsatisfactory because no one has recognized the intermediate sterile hybrids. These can reproduce vegetatively by gemmae, often forming large populations. Undetected hybrid taxa muddle species boundaries by obscuring and combining the already subtle characters that differentiate species. The obviously abortive-spored hybrids have remained undetected by most botanists. Careful field study of sympatric populations of species and hybrids over the range of the taxa, as well as examination of over 6000 herbarium specimens, has resulted in the present alignment of species boundaries and the recognition of a new species, Huperzia appalachiana, in eastern North America. First, we will review the range and characters of the common firmoss species, and then present the range and characters of Huperzia appalachiana. Looking at the three major areas of overlap (the northern Appalachians, the southern Appalachians, and the north shore of Lake Superior), the consistency of characters will be examined as well as of hybrids. The shining firmoss, Huperzia lucidula, is an eastern North American endemic of moist conifer and deciduous forests and swamps, always with some shade. Common in the northern and eastern parts of its range, it extends north of the Great Lakes up into the boreal forest, south in the Appalachians to Georgia and Alabama, west in shaded moist sandstone ravines of Missouri and Arkansas and to the edge of the deciduous woods in Minnesota. It is a robust plant of indeterminate growth habit; the individual stems continue growing indefinitely, becoming decumbent with age and eventually rotting away at their base. The leaves are in annual zones of longer leaves and shorter leaves. The shortest leaves are bud leaves produced at the end of the growing season in the form of winter bud. In H. lucidula, the leaves of the two portions are similar in size and reflexed position. The long leaves are widest beyond the middle with serrate margins and, importantly, have no stomates on the upper surface. The gemmiphores and the gemmae are relatively large. The lateral lobes of the gemmae are broadly obtuse with an apiculate tip. The spores are relatively small, averaging about 25 gm.

×Dryostichum singulare (Dryopteridaceae), a new fern nothogenus from Ontario

A previously unknown fern, here named ×Dryostichum singulare nothogen. et nothosp.nov. W. H. Wagner, has been discovered in several localities in the Georgian Bay region of Ontario, Canada. The plants are found in shaded northern hardwood forests on dolomitic outcrops. First thought to be a bizarre mutation, evidence is presented here that it is a recurring natural hybrid between Dryopteris goldiana and Polystichum lonchitis. Chromosome and spore studies indicate that the hybrid is sterile and has abortive spores; the chromosome number is 2n = 82 with no pairing at meiotic metaphase. Key words: fern, ×Dryostichum, Dryopteris, Polystichum, hybrid, Ontario.

Control Points in the Generation Cycle of Ferns

Summary The homosporous ferns, having an independent gametophytic phase, are well suited for the study of the factors controlling the life cycle. The sites of phase change can be identified as gametogenesis and sporogenesis respectively. The aspect of gametogenesis significant in relation to the change of growth appears to be oogenesis and the generation of the complex cytoplasm of the egg cell, to which the spermatozoid adds little more than a complement of Mendelian genes. The archegonium, the site of oogenesis, is surrounded by a thickened wall lacking plasmodesmata, probably impermeable to large molecules. Autoradiographs of maturing archegonia following feeding with tritiated nucleosides show that the products of lysis of the cells in the upper part of the canal move down to the egg cell, which becomes in consequence heavily labelled. In microbial cells the existence of «global regulatory networks» coming into play in response to stressful changes in the cell's environment and controlling gene activation are well established (see, for example, Slater, J. H. et al.; 1983). It would be strange if eukaryotic cells were not similarly equipped. In the particular instance of the egg cell, subjected to the assault of complex macromolecules in the intimate confines of the archegonium, we may legitimately suppose that amongst the genes activated by the regulatory network are those responsible for sporophytic growth. The presence of sporophytic messengers already present in the mature gamete is indicated by its failure to produce stable embryos when its RNA is modified during maturation by the incorporation of uridine analogues. The deactivation of the sporophytic genes in sporogenesis is envisaged as resulting from the isolation of the spore mother cells from the correlating influences of the parent sporophyte. Although there is substantial loss of cytoplasmic RNA during sporogenesis in ferns, recent work with gymnosperms (particularly Taxus) has revealed that this is not a universal phenomenon in meiotic prophase. The extent of the depolymerization may be related, possibly causally, more with the time spent in meiosis than with change of phase. Experiments with apospory have revealed isolation from the parent plant to be the common element in all instances of transition from sporophyte to gametophyte. In sporangia this is achieved by the thickened outer boundary of the tapetum and the massive and often callosed wall of the spore mother cell, in both instances conspicuously free of plasmodesmata. In the case of excised sporophytic tissue in vitro gametophytic outgrowths may appear within one or two days, but spores, lying at maturity in dry containers, must await liberation and hydration. Megasporogenesis in the heterosporous ferns takes on features of oogenesis in the homosporous. The surviving megaspore matures in a particularly rich environment to which the lysis of the abortive spores contributes. This may initiate a precocious reactivation of the sporophytic genes, and account for the simpler archegonium and egg cell in these ferns. On this view heterosporous cycles lack a pure gametophytic phase on the female side. This interpretation also holds for the heterosporous seed plants. The remarkable similarity between a linear tetrad of megaspores and an archegonium may extend beyond morphology to function, the haploid genome of the surviving megaspore being subjected to an environmental stress similar to that experienced by the genome of the egg cell of a homosporous fern. [The experimental work on which this abstract is based will be found reviewed in Sheffield, E. and Bell, P. R. Current studies of the pteridophyte life cycle. Bot. Rev. (53, 442-490 (1987)].

Detecting Abortive Spores in Herbarium Specimens of Sterile Hybrids

Uses of herbarium specimens for biosystematic research are being extended by new techniques, and those described here have been exploited only in the past few decades. Not only do dried and pressed pteridophyte specimens provide such classical gross data as geographical, ecological, and morphological, but microscopic and chemical data as well. Specimens on herbarium sheets often yield hypotheses of hybridity by morphology alone, based on the phenomenon of intermediacy (W. Wagner, 1983), in which the characters of higher plant hybrids tend to have expressions lying between those of the parental species: If one has ovate leaves, and the other lanceolate leaves, the hybrid has more or less ovate-lanceolate leaves. The hybrid's characters are not necessarily in the middle; some may be closer to one parent, some closer to the other. Probably most investigations of hybrid pteridophytes begin in the herbarium, where specimens showing interspecific intermediacy are encountered. We call such specimens because they do not fit either presumed parent. Whenever they are found we examine their spores. Interspecific pteridophyte hybrids fall into three categories as regards spore production, namely (a) sterile hybrids that have defective spores (e.g., Fig. 1), (b) fertile hybrids (either introgressant or, much more commonly, allopolyploid) that have normal viable spores, and (c) apogamous hybrids that produce unreduced spores. We are not concerned here with fertile hybrids, except for those that are apogamous (e.g., in the United States, Pteris x hillebrandii and Asplenium x heteroresiliens). Obligately apogamous hybrids can usually be recognized because some or most of the sporangia have only 32 spores that are viable; those with 64 spores show abortion (various combinations are also possible; cf. Manton, 1950). Apogamous intertaxa involve one apogamous and one sexual parent. Non-hybrid taxa with normally 32 or 16 spores per sporangium might be confused with apogamous hybrids but these can be recognized because the reduced spore number is regularized, characteristic of all sporangia, and the spores are non-abortive. Examples of 32- or 16-spored sporangia in normal sexual species are found in a wide diversity of genera (e.g., Ceratopteris, Cheilanthes, Lindsaea, Alsophila, Sadleria). Allopolyploid sexual hybrids behave like ordinary species, even though they are actually hybrids, and many of these are frequent or common; they are recognized by their spore size, usually larger than that of their parents. Not all spore abortion is associated with hybridity (see below under Additional Perspectives). Nearly all spore samples from normal species show at least some deviant spores. It is not unusual to find as much as 10-20 percent of spores

The Chromosomes of Lycopodium lucidulum

Mart.), intermediate sterile hybrids are occasionally found with either or both parents. These hybrids can be distinguished from L. lucidulum by their abortive spores and the presence of stomates on both surfaces of the leaves. The chromosome number of Lycopodium lucidulum Michx. has been reported several times from North America and India. The counts from India (Mehra & Verma, 1957; Ninan, 1958) based on Lycopodium lucidulum sensu Clark (1880), however, are actually from a distinct species, Lycopodium herterianum Kimmerle (Huperzia herteriana (Kumm.) Sen & Sen, L. sikkimense Herter 1909 non K. Muell. 1861), and not the nearctic Lycopodium lucidulum Michx. Love and Love (1958) first reported 2n = 264 for Lycopodium lucidulum from Quebec, but without an accompanying photograph, a drawing, locality data, or a voucher specimen citation. Later (1965), they reported chromosome counts from Mt. Washington, New Hampshire (without citation of voucher) of Lycopodium lucidulum and L. selago (which they made into subspecies of Huperzia selago) remarking . . it has been at last possible to make exact counts of the chromosomes of material of ssp. selago with appressed and patent leaves from Mt. Washington and also ssp. lucidula

Equisetum X litorale in Illinois, Iowa, Minnesota, and Wisconsin

The cross between Equisetum arvense L. (Field Horsetail) and E. fluviatile L. (Water Horsetail) results in the hybrid E. x litorale Kuhl. (Shore Horsetail). It is distinguished from its parents by its abortive spores and intermediate anatomical and morphological traits (Hauke, 1978). The hybrid occurs less frequently in the Midwest than hybrids between unbranched species of Equisetum. The hybrid was reported from one county in Illinois (Mohlenbrock & Ladd, 1978) and four counties in Wisconsin (Hauke, 1965). It was not reported from Iowa (Peck, 1976) nor from Minnesota (Tryon, 1954), even though both parents occur together in those states. Its recent discovery in Minnesota (Peck & Swanson, 1978) initiated a field and herbarium search that uncovered a second locality in Illinois, first and second localities in Iowa, second, third, and fourth localities in Minnesota, and eight new localities in Wisconsin. Most of these new localities are on or adjacent to the flood-plain of the Mississippi River or its tributaries. The 2-3 ha stand located in Allamakee County, Iowa, is probably the largest stand of the hybrid in North America (pers. comm. with Drs. Hauke and Wagner). Consquently, efforts were made to study the habitat, stand dynamics, and reproductive biology of the hybrid at this locality to identify factors which clarify the origin, abundance, and persistence of the hybrid in this locality. Observations also were made at 16 of the 20 localities of the hybrid in these four states, but in less detail. A summary of locality data is provided by Fig. I and by citations from herbarium vouchers (new county records indicated by *). The original Illinois locality (Lee County) is accepted from the report by Mohlenbrock & Ladd (1978); voucher citation was not given and the specimen was not seen. ILLINOIS: *Carroll Co.: Wet grounds near entrance to Mississippi Palisades State Park, N of Savannah, Wunderlin 2668 (MWI). IOWA: *AUamakee Co.: Lansing Twp.: Lansing Wildlife Refuge, 2-3 ha marsh, T99N R4W S12, Peck 78-54 (ISTC, KIRI, MICH), Farrar 78-6-3-1 (ISC), Roosa 1759 (ISTC). Banks of Mississippi River, June 1900, Orr s. n. (EMNM). *Des Moines Co.: Huron Twp.: Near pumping station No. 4, Iowa Slough, adjacent to Mississippi River, T72N R1W S4, Lammers 1542 and 2087 (IA, ISC, ISTC), Peck 78-300 (ISTC, KIRI, MICH). MINNESOTA: *Houston Co.: Mississippi River flood plain in wetlands of Crooked Creek at Reno, Peck 79-734 (KIRI, MICH, MIN, UWL). Washington Co.: Confluence of Valley Branch Creek and St. Croix River in thicket of Salix interior, Swanson 2878 (MIN, UWL). *Wabasha Co.: Weaver Bottoms, floodplain of Mississippi River, 2 mi N of Weaver, Peck 79-709 (KIRI, MICH, MIN, UWL). *Winona Co.: On floodplain of Mississippi River, W of Red Oak Island in Lake Onalaska, Peck 79-727 (KIRI, MICH, MIN, UWL). WISCONSIN: *Buffalo Co.: Nelson Twp.: Nelson-Trevino Bottoms of Mississippi River floodplain, T22N R14W S36, Peck 79-824 (KIRI, MICH, WIS, UWL). *Crawford Co.: Emergent along shore near bridge over Swamp Creek, 0.5 mi E of Lynxville on County Road B, T9N R6W S23, 8 Jul 1973, Dawson s. n. (UWL). Grant Co.: Wilderness area of Wyalusing State Park, T6N R6W S20/21, 19 June 1959, Patman s. n. (WIS). Green Lake Co.: Marsh shore on S side of Lake Puckaway, Marquette, Fassett 8799 (WIS). *La Crosse Co.: Barre Twp.: Seepage area at roadside ditch along Swamp Road,