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We investigate the relationship between the nested organization of mutualistic systems and their robustness against the extinction of species. We establish that a nested pattern of contacts is the best possible one as far as robustness is concerned, but only when the least linked species have the greater probability of becoming extinct. We introduce a(More)
It has been observed that mutualistic bipartite networks have a nested structure of interactions. In addition, the degree distributions associated with the two guilds involved in such networks (e.g., plants and pollinators or plants and seed dispersers) approximately follow a truncated power law (TPL). We show that nestedness and TPL distributions are(More)
Tapetum, orbicule, and pollen grain ontogeny in Colletia paradoxa and Discaria americana were studied with transmission electron microscopy (TEM). The ultrastructural changes observed during the different stages of development in the tapetal cells and related to orbicule and pollen grain formation are described. The proorbicules have the appearance of lipid(More)
Despite that there is some literature on pollen morphology of Rhamnaceae, studies addressing general aspects of the microsporogenesis, microgametogenesis, and anther development are rare. The aim of this paper is to describe the ultrastructure of pollen grain ontogeny with special attention to tapetum cytology in Hovenia dulcis. Anthers at different stages(More)
In Gondwanic Discaria, phenotypically intermediate individuals exist between Discaria chacaye (G. Don) Tortosa and Discaria articulata (Phil.) Miers, and between D. chacaye and related Ochetophila trinervis (Gillies ex Hook. & Arn.) Poepp. ex Miers. We studied phenology, pollinators, parental crossability, morphological features, and variation patterns on(More)
The global picture of plant reproduction at high altitudes is still diffuse due to conflicting reports (e.g., about which are the prevalent breeding systems) and incomplete geographical and taxonomic coverage of high-altitude ecosystems. This paper reports on the reproductive biology of Kentrothamnus wedellianus, a shrub inhabiting the Puna semidesert in(More)
Plant fitness strongly depends on the timing of flower production. In temperate climates most plants bloom during a relatively well-defined peak, while comparatively few species flower before or after the community peak. Since a phylogenetical signal has been shown to exist in the determination of reproductive phenology, it is of interest to identify(More)
The ultrastructure of the stigmas and styles of Colletia paradoxa, Colletia spinosissima, and Discaria americana was studied by transmission electron microscopy before and during anthesis. The purpose of the study was to contribute a full description of possible interactions between the male gametophyte and the sporophytic tissues of the stigma and the(More)
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