7: Gymnosperms
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- May 4, 2022
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The plants that would become the gymnosperms evolved xerophytic leaves to prevent desiccation in the dry air. Some would have the ability to grow wider (and thus taller) via the production of a new layer of secondary xylem (wood) each year. These plants could also produce exterior layers of dead cells, unlike the living epidermis, called bark. Together, the production of bark and wood are part of a process called secondary growth. To increase the chances of fertilization in the absence of water, gametes began to be dispersed aerially via pollen. Perhaps most importantly, the zygote and female gametophyte were surrounded in a protective coating and dispersed as seeds. Both seeds and pollen develop within structures called cones (strobili).
The fossil record shows gymnosperms diversifying in a dry period called the Permian that followed the swampy Carboniferous period. Extant groups of gymnosperms include conifers, cycads (somewhat similar in appearance to palms), gnetophytes, and a single species from the ginkgophytes, Ginkgo biloba. Of the approximately 1,000 species of gymnosperms alive today, about 600 of these are conifers.
- 7.1: Cycads
- Cycads often look fern-like, with large pinnately compound leaves. However, their leaves are xerophytic and tough. Seeds are produced in strobili that grow at the base of the vegetative leaves. Plants are dioecious, either producing a megastrobilus or microstrobilus.
- 7.2: Ginkgos
- Ginkgos are represented by a single extant species: Ginkgo biloba, a living fossil. Ginkgos have fan-shaped, deciduous leaves. Plants are dioecious, producing either paired ovules with fruit-like fleshy coverings or microstrobili.
- 7.3: Gnetophytes
- Gnetophytes are a group of strange angiosperm-like plants that are most likely derived from conifers. They have opposite leaves and produce fruit-like strobili. Plants are dioecious, producing microstrobili and megastrobili on different individuals. Similar to angiosperms, they produce vessel elements and undergo double fertilization.
- 7.4: Conifers
- Conifers are the largest group of gymnosperms. They are monoecious, producing megastrobili (seed cones) and microstrobili (pollen cones) on the same plant. They generally produce small needle- or scale-like leaves with a thick, waxy cuticle.
