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Section 24: Variations in the Positions of Reproductive Shoots in Vascular Plants
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Most treatments of reproductive morphology tend to isolate the reproductive organs from the remainder of the plant. Particular attention should be paid to the vegetative part of the shoot system and what, if any, transitions occur between the vegetative and reproductive phases of an individual shoot axis. This survey of reproductive morphology will focus on the cast of characters introduced in section one. The reproductive structures of angiosperms, the flowering plants will be discussed in the following section.
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Pteridophytes
Psilotum nudum has aerial shoots with bases that bear no appendages. Reduced leaves called enations develop further along the shoot. As the shoot becomes reproductive, the stem thickens and the enations are considered to be sporophylls (sporangium bearing leaves). The sporophylls, unlike the vegetative enations, are bifurcated. Each sporophyll subtends a synangium formed from three fused sporangia. At maturation the sporangia break open releasing a multitude of tiny spores. Following a reproductive phase, the shoots of the whisk fern may return to a vegetative state.
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Psilophyta |
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Selaginella develops sporophyll-bearing strobili at the tips of some or every stem, depending on the species and when they begin their reproductive phase. These cones can be hard to distinguish from the tips of vegetative shoots. Selaginella is heterosporous, and thus produces two kinds of spores. Each sporophyll bears a single sporangium on its adaxial surface. Megasporangia are borne by megasporophylls and microsporangia are borne by microsporophylls. Both kinds of sporangia occur in the same strobilus or cone.
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Lycophyta |
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Lycopodium is homosporous (one kind of spore). It produces sporangia on the upper surface of sporophylls. The sporophylls may or may not form an obvious strobilus.
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Equisetum is also homosporous. Sporangia are borne along the margins of sporangiophores (sporangia-bearing branches) that form on strobili. A strobilus subtended by a peduncle, or stalk, is borne on the tip of either a lateral or terminal shoot. The peltate sporangiophores protect the spore-producing sporangia. Each sporangium has a line of dehiscence located in the internal position. Because the sporangium splits open internally, the spores are not dispersed very far from the mother plant. Some species of Equisetum produce separate fertile (reproductive) shoots and vegetative shoots. The fertile shoots are achlorophyllous and depend on the vegetative shoots to provide them with photosynthates. Equisetum sylvaticum is one such species. In other species, such as Equisetum hyemale, the vegetative shoot becomes reproductive and produces an apical strobilus. Equisetum myriochaetum produces reproductive strobili on lateral shoots as well as the apical shoot.
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Sphenophyta |
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There are marked convergences in form and behavior between individual fern leaves and whole shoot segments in seed plants. The spores of ferns are produced in sporangia in clusters called sori which generally occur on the abaxial surface of the fronds. The sori may consist of small circular dots, stripes, patches, or broader areas covering nearly the entire abaxial leaf surface. A unique property of most fern leaves is that they can be photosynthetic and reproductive organs simultaneously. There are some taxa that show either a marked separation of the reproductive and assimilative functions into separate fronds (dimorphism) or the bearing of sporangia on separate pinnae (pinna dimorphism). In such spatial separation of sporangia on the frond they represent analogies with the arrangement of seed plant reproductive structures on whole shoots.
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Filicophyta |
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Gymnosperms
The gymnosperms are plants that produce seeds not enclosed in an ovary. The seeds are usually borne in (strobili) cones. Megasporangiate cones produce the megaspores (female equivalent) and microsporangiate cones produce the microspores (male equivalent). While most conifers are monoecious, Ginkgo biloba and the cycads are dioecious. The strobili of cycads consist of reduced leaves with attached sporangia that are tightly clustered into cone-like structures near the apex of the plant.
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Coniferophyta |
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The ovules of Ginkgo trees are produced in pairs at the end of short shoots. The ovules are protected within a fleshy seed coat and are not fertilized until they have fallen from the tree.
The microsporangia and megasporangia of most conifers are borne on the same tree. The microsporangiate cones are considered to be simple cones: the main axis bears microsporophylls directly. Male cones are usually produced on the lower branches of the tree. The megasporangiate strobili are complex cones: the main axis bears seed scale complexes homologous to lateral shoots subtended by bracts. The female strobili are usually produced in the upper branches, although some pines bear both male and female cones on the same branch. At the time of pollination, the scales of megasporangiate cones are spread apart but grow together once pollination has occurred. It may take up to 3 years from pollination before the seeds are released from the cone. At maturity, the cone scales separate and the winged seeds are dispersed by the wind.
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