Chapter 15. Magnoliophyta (I)
The Division Magnoliophyta in the Kingdom Plantae comprises those species of plants that were formerly classified as angiosperms and are known widely as the flowering plants. You have already studied flowers (Chapter 4), so now understand that the Division Magnoliophyta comprises all those species of plants that have flowers. For perspective: all of the plants we have read about in Section II of the Botany Study Guide up to this point do not have flowers, but certainly do have reproductive structures. We also know that flowers are the reproductive structures of the plants that bear them, and that reproductive structures are not limited to flowering plants. Thus, "flowers" are structures that distinguish plants in the Division Magnoliophyta from plants in the other divisions of the Kingdom Plantae. Observe a flower, and you know you are examining an angiosperm. However, not all angiosperms have obvious, showy flowers. You will need to consider that the structure of a flower is quite variable across all the many species of angiosperms (about a quarter million have been identified) and a few flowering plants actually seldom flower. However, angiosperm botanists put great stock in the structure of flowers as a way of classifying plants—more than any other part of a plant, the flower provides the basis for placement of a species in subtaxa (classes, orders, and families) of the Division Magnoliophyta.
The Division Magnoliophyta is split into two large classes: the Magnoliopsida and the Liliopsida.
- Read Flowering plants (The following links are included)
There are over 300 families and 250,000 species of flowering plants. The remainder of this chapter will be concerned with the dicotyledonous angiosperms (Class Magnoliopsida), leaving the monocotyledonous angiosperms to be covered in the next chapter.
Conifers are pollinated by wind, meaning they must produce a large amount of pollen grains for only a few to arrive at the female megaspore, resulting in fertilization. An advancement that allowed for higher rates of fertilization per energy expended in making pollen would surely result in that new plant type being prolific and even dominant. However, a single mutation would never result in a flower adequately adapted to spread pollen using animals. Flowers are the result of a special kind of evolution called co-evolution.
If plants reproduced and thrived with wind pollination, why did flowers evolve?
Suppose one wind pollinated plant began to have its' pollen eaten by an animal, for example a beetle. Then suppose the beetle spreads the pollen from the male cones to the female cones while looking for pollen because it can't tell the difference between the male and female cones; it searches both, spreading pollen from the male to the female in doing so. This plant has an advantage over all wind pollinated plants, because of a higher rate of fertilization. Any mutation that leads to pollen being spread by an animal soon becomes prevalent within a species, because the plant pollinated by an animal is more efficient. The plant and the animal rely on each other, one for food, the other for reproduction. Any mutation in the animal that helps pollinate the plant will become prolific through natural selection, just as mutations that favor feeding the insect or making the insect come to the flower will be dominant. This back and forth evolution results in such seemingly improbable structures as flowers that look like female moths that are pollinated by amorous male moths, or flowers and bird beaks that complement the feeding of the bird and the pollination of the flowers.
Class Magnoliopsida (dicots)
Members of the Class Magnoliopsida are defined partly on the basis of the seed or seedling having two cotyledons, most obvious at germination. But the differences between dicots and monocots are many, and we will be able to recognize most flowering plants that we encounter as belonging to one or the other class without having to dissect the seed or observe the seedling.
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