The Botanical Review

Amphitropical disjunct distributions between western North America and western South America have intrigued botanists for over a century. Here, specific examples of migration and speciation are investigated using herbaceous species from the phlox family (Polemoniaceae) as a model for considering the timing of dispersal relative to speciation. Comparative DNA sequencing reveals that, in Collomia and Navarretia, the South American species are allopolyploids, suggesting either two dispersals prior to the allopolyploidization event for each species with subsequent extirpation of the diploid progenitors from South America, or allopolyploid formation prior to dispersal with extirpation of these polyploids from North America. Divergence time estimates support a Pliestocene dispersal hypothesis and sequence data indicate that, at least in Collomia, hybridization of the diploid progenitors occurred in South America.

The modem morphological concept of the phloem is that of a tissue containing highly specialized elongated enucleate cells with permeable cytoplasm arranged in longitudinal rows and forming anastomosing strands together with the parenchymatous members of the phloem. These specialized cells—that is, the sieve-tube elements or their counterparts—are intimately connected with each other. In most plants these connections are localized in wall areas occupied by the sieve-plates or by similar structures comparable to highly developed simple-pits. The connecting strands, although varied in size, are larger than ordinary plasmodesmata and are usually associated with variable amounts of callose in the wall region that they traverse. The parenchyma cells show different degrees of ontogenetic and physiologic relation to the sieve-tube elements. The companion-cells of angiosperms are most closely associated. with the sieve-tube elements; then follow the albuminous cells of gymnosperms and finally the phloem parenchyma and ray cells. When the sieve-tube reaches maturity its protoplast is in a premortal state. The element is, therefore, rather short-lived. The companion and albuminous cells die at the same time as the sievetubes.

The first reports ofCarex hybrids in North America appeared in the middle and late 1800’s. By the early 1900’s and up to the middle 1900’s there was disagreement among the influential North American botanists regarding the extent to which hybrids existed. Those who recognized hybridization inCarex have been largely supported by recent studies, but only a small proportion of the reported hybrids are well documented. An understanding of hybridization in the genus is very important to systematic research and routine identification, as well as to the study of evolution and ecology. A total of 253Carex hybrids have been reported from North America, the greatest number of hybrids occurring in the northeast, where recent glaciation and overlapping flowering periods are probably among the causal factors. Based on current reports, hybrids are much more prevalent in subgenusCarex than in subgenusVignea. Field studies and cultivation have proven useful in determining hybrid parentage and delimiting morphological variation, respectively. Both reproductive and vegetative characters as well as anatomical and micromorphological characters have proven taxonomically useful. Cytological studies have documented meiotic irregularities inCarex hybrids and intermediate chromosome numbers. Variation in the meiotic anomalies is apparently associated with the relationship of the parents and with differences in chromosome number.Carex hybrids are largely sterile but some restoration of fertility is evident in backcrosses. Experimental hybridization, flavonoid chromatography, enzyme electrophoresis, phenological characteristics, developmental anomalies and host-parasite relationships have all been underexploited in the characterization of hybrids. A cross-referenced list of North AmericanCarex hybrids is included, along with a list of hybrids not yet reported for North America involving circumpolar species.

Ultramafic ecosystems are renowned for high endemism and habitat specialization. However, most of our understanding of ultramafic plant ecology comes from Mediterranean and temperate climes, raising questions about the generalizability of plant responses to ultramafic soils. This is especially apparent in tropical ultramafic ecosystems which exhibit a wide range of endemism and differentiation between ultramafic and adjacent non-ultramafic soils. Our objectives were two-fold: 1) synthesize our understanding of tropical ultramafic plant ecology, paying particular attention to generalities that may explain variation in endemism and habitat specialization among tropical ultramafic ecosystems; and 2) define an interdisciplinary research agenda using tropical ultramafic ecosystems as a macroecological model. We demonstrate that tropical ultramafic floras are diverse and variable in plant form and function due to the interactive effects of biogeography, climate, and edaphic properties. The variable rates of endemism, specialization, and stress tolerance traits across tropical ultramafic ecosystems have implications for the management and conservation of these diverse systems. Resumen. Los ecosistemas ultramáficos son reconocidos por su endemismo y especialización del hábitat. Sin embargo, la mayor parte de nuestra comprensión de la ecología vegetal ultramáfica proviene de climas mediterráneos y templados, lo que plantea dudas sobre la generalización de las respuestas de las plantas a los suelos ultramáficos. Esto es especialmente evidente en los ecosistemas tropicales ultramáficos que exhiben una amplia gama de endemismo y diferenciación entre suelos tropicales ultramáficos y no ultramáficos adyacentes. Nosotros teníamos dos objetivos: 1) sintetizar nuestra comprensión actual de la ecología de las plantas tropicales ultramáficas, prestando especial atención a las generalidades que pueden explicar la variación en el endemismo y la especialización del hábitat entre los ecosistemas tropicales ultramáficos; y 2) definir una agenda de investigación interdisciplinaria utilizando ecosistemas ultramáficos tropicales como modelo macroecológico. Las floras tropicales ultramáficas son diversas y variables en la forma y función de las plantas debido a los efectos interactivos de la biogeografía, el clima y las propiedades edáficas. Las tasas variables de endemismo, especialización y rasgos de tolerancia al estrés en los ecosistemas tropicales ultramáficos tienen implicaciones para el manejo y conservación de estos diversos sistemas.

Will installment of a few fences and rangers be sufficient to secure the rich orchid floral in an economically challenged area under the current rapidly changing environments? What are the research and restoration needs? We convened the Guangxi International Orchid Conservation Symposium in May 2009 in Leye county, a remote, rural area in southwestern China that is rich in orchids, to address these questions. Symposium participants examined orchid conservation issues from various angles, as reflected in this volume. Although the biological factors in orchid conservation were emphasized, social factors (i.e. protected area - people relations) were also presented here. These reviews and analyses will inform conservation workers in China as well as around the world.