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Transcript
Seed Plants – the Angiosperms – Flowering Plants The angiosperms are seed plants, similar to gymnosperms, but with some important evolutionary modifications. Flowers are reproductive organs derived from leaf-like appendages. The relationship of the accessory flower organs, petals and sepals, is obvious. The stamens and pistils can also be seen in development to originate from leaf-like structures. In the flowering plant life cycle, the male gametophyte which develops within the microspore wall into a pollen grain are even more reduced than in the gymnosperms. Its movement to the ovule is often aided by appearance and scent, attracting pollinators. The female gametophyte develops as the embryo sac, within an ovule, and within a new structure: the ovary. In pollination the pollen grain germinates on the stigma of the pistil and grows down the length of the style to the opening of the ovule. After fertilization, the embryo sac and ovule develop into the seed. A second fertilization produces a nutritive tissue, the endosperm, that surrounds the embryo. At maturity, the ovules, or seeds, are protected within the ripened ovary wall to become a fruit. The fruit, fleshy or dry, aids in dispersal. Peduncle – flower stalk Sepals – the lowermost or outermost whorls of structures, which are usually leaflike and protect the developing flower; the sepals collectively constitute the calyx. Petals – whorls of structures located inside and usually above the sepals; the petals collectively constitute the corolla. Androecium – the male portion of the plant; consists of stamens, each of which consist of a filament atop which is located an anther; inside the anthers are pollen grains which produce the male gametes Gynoecium – the females portion of the plant; consist of one or more carpels, each made up of an ovary, style, and stigma; the ovary contains ovules that contain the female gametes. The term pistil is sometimes used to refer to an individual carpel or a group of fused carpels. More information about Angiosperms Two classes of angiosperms Monocots • • • • • One cotyledon per embryo Flower parts in sets of three Parallel venation in leaves Multiple rings of vascular bundles in stem Lack a true vascular cambium (lateral meristem) Dicots • • • • • Two cotyledons per embryo Flower parts in sets of 4 or 5 Reticulate (i.e., netted) venation in leaves One ring or vascular bundles in stem Have a true vascular cambium (lateral meristem) A radially symmetrical flower Photo by Gita Ramsay Flower symmetry The sepals and petals are usually the most conspicuous parts of a flower, and a variety of flower types are described by the characteristics of the perianth (combined calyx and corolla). In regular (actinomorphic) flowers such as tulips, the members of the different whorls of the flower consist of similarly shaped parts that radiate from the center of the flower and are equidistant from each other. The flowers are radially symmetrical. In other flowers such as orchids, one or more part of at least one whorl are different from other parts of the same whorl. These flowers are generally bilaterally symmetrical and are said to be irregular (zygomorphic). A bilaterally symmetrical (irregular) flower Photo by Gita Ramsay Angiosperm life cycle Eggs from within the embryo sac inside the ovules, which, in turn, are enclosed in the carpels. The pollen grains, meanwhile, form within the sporangia of the anthers and are shed. Fertilization is a double process. A sperm and egg come together, producing a zygote; at the same time, another sperm fuses with the polar nuclei to produce the endosperm. The endosperm is the tissue, unique to angiosperms, that nourishes the embryo and young plant.