INVESTIGATIONS OF NORTH AMERICAN CYCADEOIDS: POLLINATION MECHANISMS IN CYCADEOIDEA

Wind was at first thought of as the chief pollinating mechanism in Cycadeoidea because of the open angiosperm-flower-like reconstruction of the cone presented by G. R. Wieland in 1906. Selfing seemed a more likely possibility after Delevoryas reinterpreted the cone as a closed structure in 1963. However, the existence of what was considered to be a dioecious group of species made it impossible to consider selfing as ubiquitous throughout the cycadeoids. Furthermore, only ovule-bearing trunks of the species considered to be dioecious were known; no corresponding trunks bearing exclusively microsporangiate cones had ever been found. Recent investigations of a trunk of the type heretofore considered as representative of the dioecious species have revealed that these cones were actually bisporangiate. The discovery that the bisporangiate condition was normal in the cycadeoids eliminates much of the confusion in understanding the system of pollination and allows selfing to be considered the chief means of pollination. Outcrossing probably occurred in very low frequency and could have been the result of a chance wind pollination or of a set of interactions between the cycadeoids and an unidentified animal. Determining the nature of the system of pollination in the cycadeoids makes it possible to speculate on the population structure and evolutionary potential of this important Mesozoic group of plants.     

INVESTIGATIONS OF NORTH AMERICAN CYCADEOIDS: MONANTHESIA

Delevoryas , Theodore . (Yale U., New Haven, Conn.) Investigations of North American cycadeoids: Monanthesia. Amer. Jour. Bot. 46(9): 657–666. Illus. 1959.—A study of the cycadeoid trunks from the Upper Cretaceous Mesaverde formation of northwestern New Mexico, which are in the collections of the Peabody Museum of Natural History, Yale University, has been completed. These plants had columnar trunks with persistent leaf bases and a cone in the axil of every leaf. The vascular supply of the cone is derived from the fusion of two cortical bundles which arise from two leaf traces, neither of which supplies the subtending leaf. Enough difference appears to exist between these forms and Cycadeoidea to warrant a separate generic designation. Monanthesia is the generic name used, and the single species, M. magnifica, is considered to include all the stems from the New Mexico localities. The pattern of vascularization of cones seems to suggest that they are foliar structures and that Monanthesia is probably a more advanced form than Cycadeoidea.     

INVESTIGATIONS OF NORTH AMERICAN CYCADEOIDS: EARLY OVULE ONTOGENY

Ovules on cones of the genus Cycadeoidea from the Cretaceous were found preserved at the pregametophytic stage of nucellar development. At this stage, the ovules are distinct from those of extant gymnosperms in that there is a greater degree of differentiation in both the integuments and the nucellus in Cycadeoidea. Linear clusters of three or four cells, which consistently occur in the middle of the nucellus, are interpreted to be the remains of linear tetrads of megaspores. This example of exceptional preservation allows, for the first time, a comparison of early ontogenetic events in a fossil seed with those of extant seeds.     

INVESTIGATIONS OF NORTH AMERICAN CYCADEOIDS: WELTRICHIA AND WILLIAMSONIA FROM THE JURASSIC OF OAXACA,MEXICO

Specimens of cones of members of the family Williamsoniaceae, order Cycadeoidales, from Jurassic deposits of Oaxaca, Mexico, demonstrate the first bona fide evidence of the pollenbearing Weltrichia from the Mexican Jurassic as well as provide new information on ovulebearing Williamsonia.     

KARYOTYPES OF EASTERN NORTH AMERICAN PHLOX

The karyotypes of 32 Phlox taxa were studied in many plants representative of populations found throughout eastern North America. Most plants were diploid with 2n = 14, but a few tetraploids and plants with B-chromosomes were also observed. Some species were virtually indistinguishable karyotypically, while others differed. Within and between the Subsections Speciosae, Divaricatae, and Subulatae, strikingly similar karyotypes were observed. Other Subsections differed significantly, with the Ovatae showing strong intrasubsectional variation. Increased karyotype asymmetry is correlated with morphological specialization. Where polyploidy was observed, discordant chromosome sets indicated allopolyploidy.     

INVESTIGATIONS OF ANGIOSPERMS FROM THE EOCENE OF NORTH AMERICA: A MIMOSOID INFLORESCENCE

Six examples of a spicate inflorescence from the Middle Eocene Claiborne Formation in western Tennessee have been investigated. Individual flowers are small, alternately arranged, nearly sessile, and perfect. The style protrudes 2 mm beyond the floral envelope and terminates in a slightly swollen, rounded stigma. Both the style and the stigma are hairy. Ten stamens are exserted and extend 5 mm beyond the floral envelope. Anthers are small, versatile, and dehisce longitudinally. Pollen grains are tricolporate, tectate, and are in permanent tetrahedral tetrads 32 μm in diameter. Comparison of the fossil inflorescences with those of extant families having multiple pollen grain configurations suggests that the fossil inflorescences are most closely allied to the subfamily Mimosoideae of the Leguminosae. These are the first structurally documented remains of the Mimosoideae from the Middle Eocene.     

INVESTIGATIONS OF ANGIOSPERMS FROM THE EOCENE OF NORTH AMERICA: JUGLANDACEOUS WINGED FRUITS

The winged fruits of Engelhardia have been reported in many early Tertiary floras in the northern Hemisphere. Recent systematic revisions of the extant genera, Engelhardia, Oreomunnea, and Alfaroa provide a helpful guide to the study of the fossil fruits. Fruit form, venation, nut septations and flower parts are features that separate the extant Asian Engelhardia from the American Oreomunnea. Four types of fossil winged fruits can be recognized in Middle Eocene sediments of southeastern North America: Engelhardia, Paraoreomunnea gen. n., Paleooreomunnea gen. n., and Pararengelhardia. Both Paleooreomunnea and Paraengelhardia represent extinct lines; no living forms have similar winged fruits. The fossil fruits of Engelhardia and Paraoreomunnea are similar to the fruits of extant Engelhardia and Oreomunnea, respectively. The fossil taxa reported in this paper were sympatric. The present allopatric distribution of Engelhardia and Oreomunnea is the result of Neogene regional extinctions. Engelhardia and Oreomunnea have probably been distinct genera since early Paleogene time.     

INVESTIGATIONS OF ANGIOSPERMS FROM THE MIDDLE EOCENE OF NORTH AMERICA: FLOWERS OF THE CELTIDOIDEAE

Flowers from the Middle Eocene Claiborne Formation of western Tennessee have both floral and palynological features suggestive of affinities with the subfamily Celtidoideae of the Ul-maceae. Floral size, pollen size, and exine structure suggest that these flowers are intermediate between insect-pollinated ancestors and the modern wind-pollinated Celtidoideae. Thus, they represent the first direct fossil evidence supporting the widespread notion that at least some of the Amentiferae have been derived from insect-pollinated ancestors.     

INVESTIGATIONS OF ANGIOSPERMS FROM THE EOCENE OF SOUTHEASTERN NORTH AMERICA: PHILODENDRON LEAF REMAINS

Large leaves, new to the fossil record, from the Claiborne Formation in western Tennessee have been collected and analyzed. Careful analyses of venation and cuticular anatomy indicate that these fossil leaves contain specific characters found in modern species of the genus Philodendron subgenus Meconostigma. Features of venation were taxonomically more useful than other features. Reports of fossils of the Araceae are scarce and those few fossils assigned to the family frequently have been inadequately studied. This report establishes a reliable and carefully documented occurrence of the Araceae in the fossil record, and provides information concerning the differentiation of the family in time and the distributions of the genus Philodendron.     

POLLEN MORPHOLOGY IN NORTH AMERICAN SPECIES OF TRILLIUM

The pollen morphology and tapetal membranes of 17 species of Trillium from North America have been examined by scanning and transmission electron microscopy. The pollen grains of the genus are consistently spherical and inaperturate. The exine is unstratified and ultra-thin. Five types of exine structure are recognized, namely, echinate-, corrugate-, granulate-, verrucate- and spinulate-type, based on the variations in the components of the exine. The granulate-type is the most common and is divided into six subtypes. The features of tapetal membranes also vary in relation with the variation of the exine structures. This study demonstrates the potential of palynological evidence in assessment of taxonomic relationships within the genus.