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Shepherd's Purse (Capsella) Embryos
An annual herb of the mustard family (Cruciferae or Brassicaceae), the shepherd's purse (Capsella bursa-pastoris) is a native to the Mediterranean nations. This hardy botanical has accompanied Europeans in their migrations, and it is now found in most every part of the world.
The shepherd's purse has a slender stem, clusters of tiny white flowers, and triangular seedpods that resemble old-world leather purses, giving them their common name. The herb bears flat, heart-shaped fruit pods, which, upon ripening, separate into two valves containing numerous yellow, oblong seeds. Capsella has a long history of medicinal applications and is believed to have diuretic and haemostyptic properties. A decoction yields a tannate and an alkaloid called bursine.
Some botanists consider the shepherd's purse to be carnivorous. When moistened, the seeds of this plant release an adhesive compound that can trap small aquatic invertebrates and, after their death, the decomposing remains may serve as fertilizer for the germinating plant embryos. This phenomenon is so notable that some people have attempted mosquito control by scattering shepherd's purses over lake surfaces. However, the true role of the seed mucous is in question since C. bursa-pastoris does not grow in particularly wet microclimates, leading to uncertainty as to why this plant would have such an adaptation.
Scientific investigators favor the study of the shepherd's purse as a general model of angiosperm embryogenesis because it exhibits a high degree of specificity during embryonic development, and is a good representative of the dicotyledonous flowering plants. In the early stages, the zygote divides into two cells and continues through a series of divisions delineating zones, which form the principal organs and then the tissue layers of the plant. Unlike some plants, endosperm is present in all the embryonic stages of the shepherd's purse, up to seed maturity, where it is virtually transformed into the two cotyledons. Upon maturation, the ovary develops into the fruit and the ovules become seeds. Unlike Ginkgo and Pinus, which represent well studied gymnosperm plant embryos that are straight and have multiple cotyledons, the flowering dicot (angiosperm) Capsella is characterized by its curved or bent embryo.
Cynthia D. Kelly, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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