The fossil chronicles four major periods of plant evolution. Each period was an adaptive radiation that followed the evolution of structures that opened new adaptive zones on the land.
- The first adaptation included a cuticle and jacketed gametangia that protected gametes and embryos. Vascular tissue transporting water and nutrients also developed during this time.
- The second major period of plant evolution was the diversification of vascular plants, about 400 million years ago. The earliest vascular plants lacked seeds.
- The third major period of plant evolution came with the origin of the seed. A seed consists of an embryo packaged along with a protective covering.
- The fourth major period of plant evolution was the emergence of flowering plants, 130 million years ago. The flower is a complex structure that bears seeds with protective chambers called ovaries. These plants gave rise to angiosperms.
Moss:
A mat of moss consists many plants growing in a tight pack, helping to hold one another up. The mat has a spongy quality, which enables it to retain water. Photosynthesis occurs in the upper portion of the plant. There is an alternation between haploid and diploid generations. The haploid gametophytes are the dominant generation is mosses.
Liverworts: Liverworts have bodies that develop into lobes, giving an appearance of the lobed liver of animals. The life cycle of liverworts are similar to that of mosses except liverworts can also reproduce asexually from bundles of cells called gemmae, which are bounced out of cups on the surface of the gametophytes by raindrops.
Horsetails:
Are an ancient lineage of seedless plants. There are 15 species of a single genus called Equisetum. The horsetail plant is the sporophyte generation. Meiosis occurs in the sporangia, and haploid spores are released. Horsetails are homosporous, the single type of spore gives rise to a bisexual gametophytes.
Ferns: There are more than 12 thousand species of ferns. Most live in the tropics. Each leaf of a fern, termed a megaphyll has a branched system of veins. Most ferns have leaves, commonly called fronds. that are compound, meaning each leaf is divided into several leaflets.
Conifers:
The reproductive structure of confers are cones. Pines, firs, larches, yews, junipers, cedars, cypresses, and redwoods all belong to this division of gymnosperm. Conifers dominate the vast forested regions of the northern hemisphere. Nearly all conifers are evergreens, meaning they retain their leaves year round. The needle shaped leaves of pines and firs are adapted to dry conditions. A thick cuticle covers the leaf reducing water loss. Conifer needles are megaphylls as are the leaves of all seed plants. Conifer trees are among the tallest, and oldest organisms on earth.
Flowering plants:
The flower id the reproductive organ of angiosperms. A flower is a compressed shoot with four whorls of modified leaves. The petals are brightly colored to attracted pollinators. With modifications in floral structure, many angiosperms are specialists at using specific animals for pollination.
