Experiments were established in 1982 to study the loss of viability of spotted knapweed seeds (achenes) in soil. Greater than 50 and 25% of buried seeds remained viable but dormant after 5 and 8 yr at two locations, respectively. Decline in seed viability was also measured in two natural seedbanks. Although the soil seed reserve decreased by 95% over a 7-yr period, approximately 400 000 viable seeds per ha remained, indicating that spotted knapweed seeds will last for many years in soil.

This study examined seed ultrastructure in relation to germination of North American dandelion seeds. Based on laboratory rearing observations, it was thought that the design of the pappus acts as a conduit facilitating water entry into the seed. It was hypothesized that seeds without a pappus would yield fewer seedlings and require more time to germinate than seeds with an intact pappus. Seed ultrastructure was investigated using scanning electron microscopy, while relative humidity and fungal association were explored as factors that may confer an advantage to intact seeds. Results indicate that germination for seeds lacking a pappus is 31% lower than control seeds (with an intact pappus) and that the seeds lacking a pappus require more time to germinate. Relative humidity did not differentially affect germination, and while a fungus Cladosporium cladosporioides was recovered internally, its presence neither enhanced germination nor decreased time to germination when tested by antimycotic removal. Electron micrographs revealed that (1) the pappus is hollow and (2) the pericarp of the fruit fuses with and partially encloses the pappus. Fusion of the pappus with the fruit suggests that this structure acts as a device to regulate seed hydration.

Gene flow between agricultural crops and related wild plant populations can produce hybrids which differ significantly from their wild counterparts in many life history traits, including seed traits. Seeds from wild annuals often possess significant dormancy, while cultivated varieties have been selected to germinate immediately under favourable conditions. Consequently, the dormancy characteristics of crop–wild hybrids could influence their survival, seed-bank dynamics and, ultimately, the extent to which crop genes persist in wild populations. Dormancy characteristics can be influenced by both maternal effects, as well as the genetics of a seed's embryo, which are the result of contributions from both parents. Here we focus on how maternal effects and embryo genetics affect seed dormancy in crop–wild hybrid sunflowers (Helianthus annuus). Using three laboratory experiments, we quantified the germination and dormancy of 15 crop–wild hybrid sunflower cross types, while also identifying achene characteristics that may influence the differential germination observed. We found that increased frequencies of crop alleles and the maternal effects imparted by hybridization can reduce dormancy, though the effect of increased frequencies of crop alleles was more pronounced in wild- and F1-produced than in the crop-produced achenes. The more open pericarp of the crop-produced achenes and the shorter relative distance that their radicles had to travel to germinate may explain some of the observed maternal effects. Finally, we generated hypotheses about how these results could affect survival and crop gene introgression in the field.

The myxocarpic diaspores of Artemisia taxa are characterized by the presence of mucilage cells in the achene coat, which differ between taxa and subgenera in shape and arrangement, and the ability to form a mucilage envelope after hydration. A detailed analysis of 38 taxa revealed three main achene behaviours related to achene coat structure and the modes of water retention after hydration. The most common achene coat possessed mucilage cells which formed a mucilage envelope after hydration; normally developed mucilage cells were able to produce mucilage after wetting. In taxa where the mucilage cells were not present in the achene coat, water was gathered under a loosely adhered achene coat or in the spaces between the covering trichomes. Myxocarpic diaspores were mostly observed in taxa occurring in dry habitats, whereas taxa devoid of the ability to produce mucilage grew predominately in mesotropic habitats. The ability to form a mucilage envelope was also related to the ploidy level. The shape and arrangement of the mucilage cells in the achene coat may differ between taxa and subgenera. In Tridentatae, the mucilage cells were dispersed in groups, forming a characteristic pattern. The role of myxocarpy in the dispersal and distribution of the Artemisia taxa is discussed.

Bulbostylis somaliensis Lye (Cyperaceae) is described from Somalia. It is closely related to B. hispidula (Vahl) R. Haines, but differs in its smaller achenes as well as in the shape and surface structure of the achenes. The major diagnostic characters are the non-triangular achene and the outermost cells to the pericarp, which have several papillae in each cell. Three subspecies are distinguished, viz. subsp. somaliensis, subsp. confusa Lye and subsp. microcarpa (Chiov.) Lye.