diff --git a/media/Figure_B26_01_09.jpg b/media/Figure_B26_01_09.jpg new file mode 100644 index 000000000..902319fd8 Binary files /dev/null and b/media/Figure_B26_01_09.jpg differ diff --git a/media/Figure_B26_01_09.png b/media/Figure_B26_01_09.png deleted file mode 100644 index 484d1e503..000000000 Binary files a/media/Figure_B26_01_09.png and /dev/null differ diff --git a/modules/m66572/index.cnxml b/modules/m66572/index.cnxml index 1b384fcdc..84ba08edc 100644 --- a/modules/m66572/index.cnxml +++ b/modules/m66572/index.cnxml @@ -20,10 +20,10 @@ Both seeds and pollen distinguish seed plants from seedless vascular plants. These innovative structures allowed seed plants to reduce or eliminate their dependence on water for gamete fertilization and development of the embryo, and to conquer dry land. Pollen grains are male gametophytes, which contain the sperm (gametes) of the plant. The small haploid (1n) cells are encased in a protective coat that prevents desiccation (drying out) and mechanical damage. Pollen grains can travel far from their original sporophyte, spreading the plant’s genes. Seeds offer the embryo protection, nourishment, and a mechanism to maintain dormancy for tens or even thousands of years, ensuring that germination can occur when growth conditions are optimal. Seeds therefore allow plants to disperse the next generation through both space and time. With such evolutionary advantages, seed plants have become the most successful and familiar group of plants. Both adaptations expanded the colonization of land begun by the bryophytes and their ancestors. Fossils place the earliest distinct seed plants at about 350 million years ago. The first reliable record of gymnosperms dates their appearance to the Pennsylvanian period, about 319 million years ago (). Gymnosperms were preceded by progymnosperms, the first naked seed plants, which arose about 380–390 million years ago. Progymnosperms were a transitional group of plants that superficially resembled conifers (cone bearers) because they produced wood from the secondary growth of the vascular tissues; however, they still reproduced like ferns, releasing spores into the environment. At least some species were heterosporous. Progymnosperms, like the extinct Archaeopteris (not to be confused with the ancient bird Archaeopteryx), dominated the forests of the late Devonian period. However, by the early (Triassic, c. 240 MYA) and middle (Jurassic, c. 205 MYA) Mesozoic era, the landscape was dominated by the true gymnosperms. Angiosperms surpassed gymnosperms by the middle of the Cretaceous (c. 100 MYA) in the late Mesozoic era, and today are the most abundant and biologically diverse plant group in most terrestrial biomes.
- + -Plant timeline. Various plant species evolved in different eras. (credit: United States Geological Survey) Figure modified from source.
+Plant timeline. Various plant species evolved in different eras. (credit: modification of work by United States Geological Survey)
Evolution of Gymnosperms The fossil plant Elkinsia polymorpha, a "seed fern" from the Devonian period—about 400 million years ago—is considered the earliest seed plant known to date. Seed ferns () produced their seeds along their branches, in structures called cupules that enclosed and protected the ovule—the female gametophyte and associated tissues—which develops into a seed upon fertilization. Seed plants resembling modern tree ferns became more numerous and diverse in the coal swamps of the Carboniferous period.