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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)
BACKGROUND AND AIMS Sunflower cultivars exhibit a wide range of oil content in the mature achene, but the relationship between this and the dynamics of oil deposition in the achene during grain filling is not known. Information on the progress, during the whole achene growth period, of the formation of oil bodies in the components of the achene and its(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)
This is the first report of the ultrastructure of the stigma and style during and after anthesis in Helianthus annuus L. using light and transmission electron microscopy. The stigma is bifid with unicellular papillae. There is no secretion of lipids, carbohydrates or proteins at anthesis. The style is semisolid in the upper portion, closer to the stigma,(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)
Bipartite graphs have received some attention in the study of social networks and of biological mutualistic systems. A generalization of a previous model is presented, that evolves the topology of the graph in order to optimally account for a given contact preference rule between the two guilds of the network. As a result, social and biological graphs are(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)
Various aspects of the reproductive biology of the perennial hermaphroditeFrangula alnus Mill. were studied in two populations located in the province of Cádiz, southern Spain. Flowering extends from the second week of May to early July. The small, whitegreenish, entomophilous flowers are incompletely protandrous and last 8–10 days, but pollen transfer(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)