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 <para id="fs-idp39544208">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 (1<emphasis effect="italics">n</emphasis>) 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.</para>
 <para id="fs-idp49354208">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 (<link target-id="fig-ch26_01_01"/>). Gymnosperms were preceded by progymnosperms, the first naked seed plants, which arose about 380–390 million years ago. <emphasis effect="italics">Progymnosperms</emphasis> 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 <emphasis effect="italics">Archaeopteris</emphasis> (not to be confused with the ancient bird <emphasis effect="italics">Archaeopteryx</emphasis>), 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.</para>
 <figure id="fig-ch26_01_01"><media id="fs-idm7582800" alt="The image depicts a branching diagram (a tree-like structure) where the branches spread horizontally from a main ancestor on the left. On the right, organism names are labeled. The cascading branching starts with a single stem at the upper left corner labeled ‘Ancestral green algae’. This has a branching point giving rise to three horizontal branches. The top two branches extend all the way to the right and are labeled ‘Coelochaetes (green algae)’ and ‘Charophytes (green algae) from top to bottom. The third branch is a short branch labeled ‘Embryophytes’ on the left. It branches into two new branches. The top branch extends all the way to the right and is labeled ‘Marchatiophyta (liverworts)’. The bottom branch divides into two additional branches, the top labeled ‘Anthrocerophyta (hornworts)’. The bottom branch splits into two branches, the top labeled at the right ‘Bryophyta (mosses)’. The liverworts, hornworts, and mosses are collectively labeled Bryophytes. The bottom branch splits into additional two branches, the top labeled at the right ‘Aglaophyton (extinct)’. The bottom splits into two branches, labeled ‘Rhynopsida (extinct)’, and one branch which splits again. The top branch splits into several branches which all end in several organisms collectively labeled ‘Lycophytes’. The order of organisms in this group from top to bottom is ‘Drepanophycales (exctinct); Lycopodiophyta (club moses)’; Protolepidodendrales (exctinct); Selaginellales (spike mosses); Isoetales (quillworts); Zosterophyllopsida (extinct). The bottom branch splits in two additional branches, the first labeled ‘Psilophyton (extinct)’, and the bottom splits again. The top branch gives rise to two branches that are labeled ‘Equisetopsida (horsetails)’; and ‘Polypodiopsida (ferns)'. The Psilophyton, horsetails, and ferns are collectively labeled 'Monilophytes'. Finally the bottom branch splits into two branches, the top labeled ‘Gymnosperms’ and the bottom 'Angiosperms’. These are collectively labeled ‘Spermatophytes’.">
-<image mime-type="image/png" src="../../media/Figure_B26_01_09.png" width="380"/>
+<image mime-type="image/jpg" src="../../media/Figure_B26_01_09.jpg" width="380"/>
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-<caption>Plant timeline. Various plant species evolved in different eras. (credit: United States Geological Survey) Figure modified from source.</caption></figure><section id="fs-idp1993232">
+<caption>Plant timeline. Various plant species evolved in different eras. (credit: modification of work by United States Geological Survey)</caption></figure><section id="fs-idp1993232">
 <title>Evolution of Gymnosperms</title>
 <para id="fs-idp106419072">The fossil plant <emphasis effect="italics">Elkinsia polymorpha</emphasis>, a "seed fern" from the Devonian period—about 400 million years ago—is considered the earliest seed plant known to date. Seed ferns (<link target-id="fig-ch26_01_02"/>) 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.</para>
 <figure id="fig-ch26_01_02">