1. Angiosperm Evolution and Diversity
Angiosperms
Angiosperm Evolution and Diversity
Flowering plants, or angiosperms, are one of the two main groups of seed plants.
Angiosperms are characterized by seeds that develop enclosed within ovaries, a trait that distinguishes them from the gymnosperms, the other major group of seed plants, in which seeds are "naked."
The flower contains the reproductive organs. It can contain just a male organ, just a female organ, or both.
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2. Angiosperms
Figure 1 Angiosperm flowers contain male and female reproductive organs.
The majority of angiosperm flowers contain both male parts (stamens) and female parts (carpel). The petals are actually modified leaves.
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3. Angiosperms
Angiosperms evolved from gymnosperms and diversified during the Cretaceous period.
Angiosperms are the dominant and most diverse plants on Earth. They represent nearly 90% of all terrestrial plants and include more than 250,000 species.
Having enclosed seeds is important but is not the only reason for angiosperms' success. Angiosperms tend to have more efficient vascular systems and more efficient leaves than gymnosperms do.
Many of angiosperms' roots and stems are modified for nutrient storage, and they undergo a unique kind of fertilization.
The oldest confirmed angiosperm fossils are of early-Cretaceous origin and are approximately 125 million years old.
One of the earliest confirmed angiosperm fossils, Leefructus mirus, represents an early (basal) eudicot possibly related to buttercups
One of the earliest confirmed angiosperm fossils, a fossil from a genus named Archaefructus, represents a basal aquatic angiosperm possibly related to water lilies.
Both Leefructus mirus and Archaefructus were herbaceous and both had fully enclosed seeds.
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4. Figure 2 Leefructus — the first known dicot.
Angiosperms
Figure 2 Leefructus — the first known dicot.
This spectacularly preserved fossil indicates that Leefructus was a relatively complex angiosperm with a single flower and enclosed seeds. (Scale bar, 1 cm)
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5. Angiosperms
Scientists classify angiosperms using fossil and molecular characteristics.
Among scientists who think the earliest angiosperms were terrestrial, there are two competing hypotheses: the Magnoliid Hypothesis and the Paleoherb Hypothesis.
Scientists subscribing to the Magnoliid Hypothesis think the basal angiosperms grew in the manner of trees or shrubs, much like a modern-day magnolia tree, with large flowers, slow life cycles, long woody stems, and a leafy crown.
The Paleoherb Hypothesis contends that the earliest angiosperms were short, herbaceous plants with small flowers, rapid life cycles, soft stems, and leaves growing from low on the plant.
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6. Figure 3 A phylogenetic tree based on molecular evidence.
Angiosperms
Figure 3 A phylogenetic tree based on molecular evidence.
Amborella lies at the base of this phylogenetic tree. (The size of the triangles represents the relative abundance of each phylogenetic group on Earth today.)
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7. Angiosperms
Scientists classify angiosperms using fossil and molecular characteristics.
Amborella flowers are unique egg-containing structures that some scientists suggest might be a kind of "missing link" connecting angiosperms and gymnosperms.
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8. Phylogenetic Studies Can Yield Different Results
Angiosperms
BIOSKILL
Phylogenetic Studies Can Yield Different Results
Most phylogenetic analyses agree with the results of the original 1999 paper placing Amborella at the base of the angiosperm family tree.
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9. Figure 4 Comparing phylogenetic trees.
Angiosperms
Figure 4 Comparing phylogenetic trees.
The data used to make these trees were identical save for the addition of a species of orchid (Oncidium) to the tree on the right. (The monocots are highlighted in green, Amborella in orange.)
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10. Future perspectives and open questions.
Angiosperms
Future perspectives and open questions.
The study of gene function in regard to angiosperm evolution is part of a wider approach known as evo-devo. Evo-devo studies are based on the observation that many developmental genes are evolutionarily conserved.
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11. Angiosperms
Flowers and fruits are the major reproductive adaptations of angiosperms.
The stamen, containing the anther and filament, is the male part of a flower.
The anther contains pollen, the male gametophytes. Pollen is a fine, powdery material, often yellowish in appearance, composed of tiny grains.
Each pollen grain contains two cells, one (the generative cell, which becomes the sperm) embedded within the other (the tube cell).
The filament is a stalk that supports the anther, holding it away from the female part of the flower to prevent self-fertilization.
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12. Figure 5 Male reproductive organs.
Angiosperms
Figure 5 Male reproductive organs.
The stamen is supported by the filament. The anther contains the pollen, which in turn contains the male gametes.
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13. Angiosperms
Flowers and fruits are the major reproductive adaptations of angiosperms.
The female parts of the flower-collectively, the carpel-consist of the stigma, the style, the ovary, and ovules.
The stigma is a usually sticky receptive apex where pollen attaches.
The tube cell in pollen burrows down into the style, a typically slender part of the female organ that supports the stigma.
At the base of the style is the ovary, which encloses and protects the ovules.
Each ovule contains an egg cell. When the pollen fertilizes an egg in the ovule, a seed begins to develop. At the same time, the ovary wall starts to form the fruit.
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14. Figure 6 Female reproductive organs.
Angiosperms
Figure 6 Female reproductive organs.
The pistil is made up of the stigma, s-t-y-l-e, and ovary. The ovules are embedded in the ovary and, when fertilized by the male sperm, become the seeds. The ovary becomes the fruit.
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15. Angiosperms
Most angiosperms rely on insects or other animals for pollination and seed dispersal.
Gymnosperms rely on wind to carry their pollen to other plants. They produce an enormous amount of pollen to ensure success.
Most angiosperms spend less energy on pollen production than gymnosperms do because they have evolved methods of selective pollination.
Angiosperms make showy flowers and sweet nectar. Insects and other animals are attracted to the flowers and, as they move from flower to flower, they transfer the pollen, serving as inadvertent pollinators.
Pollinators enhance angiosperm genetic variability.
Flowers attract pollinators through three primary strategies: reward, deception, and entrapment. Usually, these strategies involve petals.
Petals are modified leaves, often brightly colored, surrounding the reproductive organs that provide visual or odorous cues for pollinators.
The corolla is the flower's entire group, or whorl, of petals.
When the flower is in its bud phase, the petals and other organs are protected by sepals, which are located just below the petals.
Reward strategies lure a pollinator with the promise of nectar. Nectaries are usually at the base of the flower, obliging the pollinator to brush against the stamens and carpels as they retrieve it
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16. Figure 8 Same flower; different light.
Angiosperms
Figure 8 Same flower; different light.
The Mimulus flower on the right is identical to the flower on the left except it is shown in ultraviolet light, as a bee might see it. The distinct dark spots guides bees to the flower’s nectary.
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17. Angiosperms
Most angiosperms rely on insects or other animals for pollination and seed dispersal.
Flowers that use deception as a pollination strategy trick insects into visiting them. In some orchids, the flower's petals are shaped so as to resemble the female insect.
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18. Figure 9 Pollination by deception.
Angiosperms
Figure 9 Pollination by deception.
Orchid flowers such as this (Cypripedium calceolus) resemble female insects so males will help pollinate the flower. Many orchids are dependent on a single pollinating species. Most plant-pollinator associations are more general.
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19. Angiosperms
Most angiosperms rely on insects or other animals for pollination and seed dispersal.
Entrapment is the third pollination strategy used by flowers. Flowers promise the reward of nectar but trap the insect in such a way that it can exit only after brushing against the stamen and carpels.
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20. Figure 10 Pollination by entrapment.
Angiosperms
Figure 10 Pollination by entrapment.
The pink “slipper” of this orchid traps bumblebees that land on it. Once a bee has pollinated the flower, the slipper droops so that no other bee can enter.
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21. Angiosperms
Most angiosperms rely on insects or other animals for pollination and seed dispersal.
Fruits protect the seeds that develop within them. However, when the seeds mature, most fruits advertise their availability to animals with the promise of nutrition and sweetness. Animals eat the fruit and deposit the seeds in their feces.
Scientists place fruits into four categories: simple, aggregate, multiple, and accessory.
Simple fruits such as the pea, bean, and tomato are derived from the ovary of a single carpel.
Aggregate fruits such as blackberries are derived from many ovaries from numerous carpels, each of which gives rise to a "fruitlet" that combines to form the fruit.
Multiple fruits form from ovaries in flower clusters, called inflorescences.
Accessory fruits contain tissue derived from parts of the flower other than the ovary.
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22. Figure 11 Apple development.
Angiosperms
Figure 11 Apple development.
The apple is considered an accessory fruit because it contains tissue from parts of the flower other than the ovary.
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