5.3 Class Magnoliopsida – flowering plants - Cambridge University ...

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224 ORDERING THE PATHS OF DIVERSITY Figure 5.54. Models of the origin of the angiosperm flower. ago. The flowering-plants probably originated in the tropics; extant primitive families are tropical or subtropical and early fossils show no adaptations to temperate conditions. There have been a number of theories about the origin of flowers, homologising in different ways floral structures with the reproductive structures of other plants. One theory, the anthophyte theory, related flowers to the bisexual flower-like axes present in groups such as the extinct Bennettitales and living Gnetidae. Two contrasting theories are the euanthium theory, which derives a flower from a uniaxial cone bearing both micro- and megasporophylls, and the pseudanthium theory, which derives the hermaphrodite flower from a complex inflorescence of unisexual male and female flowers. However, these different theories place undue emphasis on a fundamental distinction between organ types. It has also been suggested that a flower-like structure could have arisen by a change of sex of some of the microsporophylls of a male cone, a process called gamoheterotropy. This process is not hard to imagine in plants that are hermaphrodite and where the determination of sex is a developmental phenomenon that is only rarely associated with sex chromosomes. A similar transfer of sex has been proposed in the more recent evolution of the maize cob. This theory has received recent support from a study of the genes of reproductive development. The gene determining the sex of floral organs in flowering plants is a homologue of a gene active in the male axis of conifers but not the female axis. The first known leaves of flowering plants appear in fossils dated at 125 Ma and the first fossil inflorescence has been dated at 120 Ma. fossil has a mosaic of characteristics that are found in a range of basal groups of flowering plants. This has a female inflorescence, with a bract and two bracteoles at the base of female flowers that lack a perianth (achlamydeous flowers). The flowers are effectively just tiny pistils, less than 1 mm in diameter. 5.3.3 The evolutionary radiation of flowering plants There are many threads to the evolutionary diversification of flowering plants: specialisation for pollination, including a reversal to wind pollination as well as more and more bizarre adaptations to attract specialist animal pollinators specialisation in fruit and seed dispersal, such as fleshy or otherwise elaborated fruits, and the evolution of seed dormancy adaptations for seedling establishment and growth in competition with other plants specialisation for growing in different habitats from aquatic to arid terrestrial habitats, surviving heat, cold and low light levels, or growing as epiphytes and parasites adaptations conferring resistance to or prevention of herbivory
Insect pollination is closely associated with the origin and subsequent diversification of flowers. However, it is important to remember that insect pollination is associated with several other groups of seed plants, both living and extinct: Bennettitales, Gnetales, Cheirolepidaceae (extinct conifers), Cycadales and Medullosales (seed ferns). Insects grew in diversity with the origin of seed plants in the Late Devonian and this increase of diversity with the origin of flowers is just part of a continuous trend. Indeed there is some evidence to indicate a temporary decline in insect diversity as flowering plants became more abundant in the Cretaceous. Nevertheless, flowers diversified in parallel with particular groups of pollinator specialists, the bees (Apoidea/Apidae), the pollen wasps (Vespidae: Masarinae), brachyceran flies (Acroceridae, Apioceridae, Bombyliidae, Empididae, Nemestrinidae, Stratiomyidae and Syrphidae) and the moths and butterflies (Lepidoptera). The evolution of a bisexual reproductive axis was a crucial event. Several trends in floral evolution can be discerned. Primitive flowerseitherlackaperianthorhaveoneinwhichthereisasingle whorl of tepals. In the perianth there has been a trend from having a perianth in which distinct whorls are not clearly differentiated to clear specialisation of a distinct calyx and corolla. The calyx protects against drought, temperature shock and predatory insects and the corolla attracts and controls pollinators. Both calyx and corolla may have been derived from tepals, but it is likely that in some groups, such as the buttercups, the petals originated from stamens, to which they are anatomically similar. In the peony, Paeonia, there is a gradual transition from leaves, through modified leaves on the flowering stem called bracts, into the perianth with parts at first sepal-like and then petal-like. Generally there has been a trend for the greater integration of the floral parts with greater precision in number and placement as flowers have become specialised to particular patterns of pollination. An important aspect of the diversification of flowering plants and their evolutionary success has been their vegetative flexibility. They have evolved into a bewildering range of forms through the activity of sub-apical and intercalary meristems. Flowering plants also have a greater capacity for elongation of cells, including root hairs and trichomes. One of the most important vegetative specialisations has been their possession of vessels in their wood, permitting more efficient water transport and hence greater photosynthetic rates, fast growth rates and earlier maturation. There have been significant adaptations in seasonal habitats; for example, although a few nonflowering plants are deciduous this has been a particular floweringplant trait that has adapted them to high latitudes or seasonally-dry environments. One aspect to the burgeoning biotic diversity of flowering plants from the Cretaceous onwards was their great expansion of chemical diversity. New ranges of what have been called secondary compounds provided attractants for pollinators or seed dispersers, and repellants and toxic compounds to inhibit herbivores. 5.3 CLASS MAGNOLIOPSIDA – FLOWERING PLANTS 225 (a) (b) Figure 5.55. Specialisation in the perianth in the Asparagales: (a) Belamcanda; (b) Kniphofia.
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Page 21 and 22: (all Faboideae/Popilionoideae). All
Page 23 and 24: tri-foliolate leaves. Many of these
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Page 27 and 28: apiales (umbels and ivy) There are
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5.3 Class Magnoliopsida – flowering plants - Cambridge University ...

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