1. Gymnosperm Evolution and Diversity
Gymnosperms
Gymnosperm Evolution and Diversity
The term gymnosperm literally means "naked seed," referring to the seeds' lack of ovaries that form around and encase the seeds in fruit, like with angiosperms.
Gymnosperms form seeds directly on modified leaves knows as sporophylls. In most cases, these sporophylls form cones.
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2. Gymnosperms
Gymnosperms evolved from early seed plants in the Carboniferous Period.
Seed plants evolved from seedless, nonvascular plants related to modern ferns.
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3. Figure 1 Seed plant evolution through geologic time.
Gymnosperms
Figure 1 Seed plant evolution through geologic time.
Gymnosperms evolved from early seed plants. Transitional forms appear throughout the Carboniferous period. Gymnosperm evolution predates angiosperms by about 145 million years.
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4. Gymnosperms
Gymnosperms evolved from early seed plants in the Carboniferous Period.
Throughout the warm, wet Carboniferous period, from 354 to 290 Ma, gymnosperms remained minor players in lush ecosystems dominated by seedless vascular plants.
Environmental conditions changed dramatically during the Permian period, culminating in the formation of the supercontinent Pangaea.
As rainfall decreased in quantity and regularity, dry land replaced swamps, and the evolutionary advantage shifted decisively to gymnosperms, whose land adaptations conferred greater fitness in the new environment.
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5. Gymnosperms Figure 2 Pangaea.
During the Permian, the continents merged forming the supercontinent Pangaea.
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6. Gymnosperms
Gymnosperms evolved from early seed plants in the Carboniferous Period.
These land adaptations included the evolution of pollen, which liberated plant reproduction from the requirement of a water pathway between male and female gametophytes.
Pollen is the entire male gametophyte made up of less than a handful of cells and encased inside a layered protective coating. This adaptation allowed the male gametophyte to survive on dry land and opened the door for wind dispersal and insect pollination.
Another critical land adaptation was the seed, which includes the plant embryo, a protective coating, and a food source.
A gymnosperm's female gametophyte develops inside a protective structure, called an ovule. When pollen reaches an ovule, the male gametophyte essentially sprouts, forming a tube-like structure called a pollen tube, which grows through the ovule's protective coating.
Once fertilization occurs, the zygote develops within the protective coating of the ovule. The entire structure develops into a seed.
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7. Figure 3 Seed structure and development in gymnosperms.
The seed develops from the female ovule after fertilization. The food supply develops from haploid tissue of the female gametophyte. The diploid embryo begins the sporophyte generation.
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8. Gymnosperms
Gymnosperms evolved from early seed plants in the Carboniferous Period.
Many gymnosperms have a woody coating outside the vascular cambium, commonly called bark, another evolutionary adaptation of seed plants.
Inner bark contains phloem produced by the vascular cambium, which serves the function of sugar transport.
Outer bark includes lateral meristems known as cork cambia as well as tissue produced by cork cambia called periderm.
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9. Future perspectives and open questions.
Gymnosperms
Future perspectives and open questions.
Evolutionary relationships among the seed plants are murky, mainly as a result of the poor preservation of plant tissue in the fossil record.
Researchers do agree, however, that the four existing groups are monophyletic.
• Cycadidae (cycads) are often commonly referred to as palms, although true palms are distantly related angiosperms. • Ginkgoidae (gingko) contains a single surviving species, Gingko biloba. • Gnetidae (gnetophytes) share some superficial morphological characteristics with early angiosperms but are most closely related to conifers. • Pinidae (conifers) is by far the largest group with approximately 600 species.
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10. Gymnosperms Figure 5 Ginkgo biloba.
Flat, fan-shaped leaves and fleshy seeds (not shown) make the ginkgo tree seem like an angiosperm. However, it does not form true fruit or flowers.
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11. Gymnosperms Figure 6 Ephedra.
This E. equisetina has primitive flower-like structures, but they most likely evolved separately from the flowers of angiosperms.
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12. Figure 7 Cycads in their natural habitat.
Gymnosperms
Figure 7 Cycads in their natural habitat.
These Macrozamia communis grow in eucalyptus forests of Southeastern Australia.
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13. Conifers are the largest group of extant gymnosperms.
Conifer leaf adaptations increased their fitness in the Permian environment and help them survive in extreme environments today.
The needle shape of most conifer leaves minimize surface area, reducing water loss from the plant.
Conifer leaves produce a waxy cuticle that further inhibits water loss.
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14. Gymnosperms Figure 8 Juniper branch.
The “berries” of the Juniper tree are actually ovule-producing cones that have soft, compressed scales.
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