Justin K. Williams

A further evaluation of Echites sect. Yucatanense (Apocynaceae) with additional notes on the genus

The speciesEchites turrigera Woodson has been considered a synonym ofE. yucatanensis Millsp. Evidence presented here shows that the two species are distinct. In addition,Echites parviflora Sessé & Moç is presented here as a synonym ofE. woodsoniana Monach.

A New Combination in Mexican Mandevilla (Apocynaceae Subfamily Apocynoideae) II

Abstract: Echites exilicaulis Sessé & Moç. is shown to belong in the genus Mandevilla and that it is a species distinct from Mandevilla andrieuxii (Müll. Arg.) Hemsl. Accordingly Mandevilla exilicaulis (Sessé & Moç.) J. K. Williams is proposed.

Host Cues Mediate Growth and Establishment of Oak Mistletoe (Phoradendron leucarpum, Viscaceae), an Aerial Parasitic Plant

ABSTRACT  The oak mistletoe (Phoradendron leucarpum, Viscaceae) is well-documented to exhibit preference for a few potential host species in a given locality, even when many potential host species are present. In trying to explain this distribution, we examined the mechanisms by which mistletoe seedlings recognize potentially suitable hosts in the Piney Woods ecoregion of east Texas. An initial survey of patterns of infection on the campus of Sam Houston State University revealed that water oak (Quercus nigra) was host to nearly half of the mistletoes observed, despite comprising less than 15% of trees surveyed. Field experiments demonstrated that light, host physiochemistry, and volatiles released from potential host trees serve as cues affecting the viability and establishment of mistletoe seedlings. These results provoked further study in controlled laboratory settings, in which it was demonstrated that chemical compounds in the bark of local host trees (compared to trees that serve as hosts elsewhere, but not in our survey) induce significantly although slightly greater seedling viability. Establishment of haustoria depended only on the presence of these chemicals, regardless of host species. Importantly, we demonstrated that three common monoterpenes, limonene, β-myrcene, and β-phellandrene induce a positive growth response of mistletoe radicles. These results taken together suggest a model to explain local host preference in P. leucarpum, in which covariation between mistletoe fruit maturity and monoterpene production by hosts determines the distribution of successful haustorial establishment.

A Geographic Information Systems (GIS)-based analysis of modern South African rodent distributions, habitat use, and environmental tolerances

Goals of this study were to: (1) develop distributional maps of modern rodent genera throughout the countries of South Africa, Lesotho, and Swaziland by georeferencing museum specimens; (2) assess habitat preferences for genera by cross-referencing locality position with South African vegetation; and (3) identify mean annual precipitation and temperature range where the genera are located. Conterminous South Africa including the countries of Lesotho and Swaziland Digital databases of rodent museum specimens housed in the Ditsong National Museum of Natural History, South Africa (DM), and the Division of Mammals, National Museum of Natural History, Smithsonian Institution, United States (NMNH), were acquired and then sorted into a subset of specimens with associated coordinate data. The coordinate data were then used to develop distributional maps for the rodent genera present within the study area. Percent habitat occupation and descriptive statistics for six climatic variables were then determined for each genus by cross-referencing locality positions with vegetation and climatic maps. This report presents a series of maps illustrating the distribution of 35 rodent genera based on 19,471 geo-referenced specimens obtained from two major collections. Inferred habitat use by taxon is provided for both locality and specimen percent occurrence at three hierarchical habitat levels: biome, bioregion, and vegetation unit. Descriptive statistics for six climatic variables are also provided for each genus based on locality and specimen percent incidence. As rodent faunas are commonly used in paleoenvironmental reconstructions, an accurate assessment of rodent environmental tolerance ranges is necessary before confidence can be placed in an actualistic model. While the data presented here represent only a subset of the modern geographic distributions for many of the taxa examined, a wide range of environmental regimes are observed, suggesting that more research is necessary in order to accurately reconstruct an environmental signature when these taxa are found in the fossil record.

(Vol.13, 2010-12) The Status of Fraxinus Papillosa (Oleaceae): SEM Study Of Epidermal Features

The abaxial leaf surfaces of Fraxinus in the southwestern USA and northwestern Mexico are sometimes white-glaucous and have been described as ‘‘papillose.’’ Individuals exhibiting this morphology have been identified as F. papillosa, a species reported from Arizona, New Mexico, Texas, and Mexico. In the original description of F. papillosa, its leaf epidermal structure was compared to F. americana L., but the abaxial surface morphology of F. papillosa is distinctly different from that of F. americana. In the latter, a cuticular reticulum overlays the epidermis and partially obscures it, while the abaxial surface of F. papillosa is similar to that of F. velutina in its lack of a cuticular overlay. In F. papillosa each epidermal cell is abruptly convex or folded upward into a papilla-like structure, compared to the relatively flat-surfaced cells of F. velutina—intergradation occurs but whether the two are appropriately considered conspecific remains to be determined. SEM photographs of the abaxial leaf surface morphology of F. papillosa, F. velutina, and other species that are similar to F. papillosa are provided.