Plant Evolution

Introduction to the Green Plants Land plants first appeared in the Ordovician (~460 million years ago) Today, with more than 250,000 species, they are second in size only to the insects. We now know that plants, like all living organisms, had aquatic ancestors. A specific group of freshwater green algae are the closest relatives to the land plants. Land plants first appeared in the Ordovician (~460 million years ago) but did not begin to resemble modern plants until the Late Silurian. By the close of the Devonian, about 360 million years ago, there were a wide variety of shapes and sizes of plants, including tiny creeping plants and tall forest trees. Today, with more than 250,000 species, they are second in size only to the insects. We now know that plants, like all living organisms, had aquatic ancestors. A specific group of freshwater green algae are the closest relatives to the land plants. The story of plant evolution is therefore inseparably linked with their progressive occupation of the land and their increasing independence from water for reproduction.

Charophytes Charophytes, a group of fresh water green algae, are the closest relatives to the land plants. Green algae differ from plants in many ways. they don't have a specialized transport or support systems. green algae do not have true leaves green algae lack cuticles (a waxy layer on the outer wall of epidermal cells) and stomata (specialized cells for gas exchange). Like the land plants, green algae contain two forms of chlorophyll (a and b), which they use to capture light energy to make sugars Green algae differ from plants in many ways. Because they live in the water, they don't have a specialized transport or support systems. Their bodies are supported by the water, and almost all of the cells photosynthesize and have access to the nutrients present in the water. Therefore, transport of nutrients is not necessary. Even though they photosynthesize, green algae do not have true leaves (which are characterized by the presence of vascular tissue). Additionally, green algae lack cuticles (a waxy layer on the outer wall of epidermal cells) and stomata (specialized cells for gas exchange). 2. Chara, a green alga.

Early land plants The early land plants, possessed two important features that allowed them to live on land: 1) a waxy cuticle to protect against desiccation (drying out) and 2) protection of gametes and embryos in a protective jacket made of cells. Even with these adaptations, early land plants were still closely tied to water. They needed water for reproduction and photosynthesis. Still had no vascular system.

Bryophytes (The mosses, liverworts and hornworts) Bryophytes don't receive as much attention as flowering plants, ferns, or conifers because most bryophytes are small and inconspicuous. They have no vascular tissue (no true xylem or phloem) to lend them structural support, nor do they have true leaves or showy flowers. This does not mean that bryophytes are not important; mosses in particular, play important roles in prevention erosion along and insulating the arctic permafrost.

Plant Life Cycle Plant life cycles alternate between a sporophyte phase, which produces spores, and a gametophyte stage, which produces gametes. A zygote divides by mitosis and grows into a mature sporophyte, or spore- producing plant. A spore divides by mitosis and grows into a mature gametophyte, or gamete-producing plant.

Alternation of Generations Sporophyte is the diploid multicellular stage of plants and algae that undergo alternation of generations, with each of its cells containing two sets of chromosomes Gameotphyte is the haploid, multicellular stage of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes. In bryophytes, the gametophytes are nutritionally independent of the sporophytes and the sporophytes are either completely or partially dependent on the gametophytes. Sperm are free swimming and require water to reach the egg.

The second major period of plant evolution: The diversification of vascular plants Plants, developed an efficient vascular systems, consisting of xylem and phloem, solved the problem of water and food transport throughout the plant body. Plants also started to synthesize lignin (a plant polymer), which is incorporated into the cell wall. Lignin adds rigidity to cell walls, making it possible for plants to reach great heights. The reproductive systems of plants were also changing. The gametophyte stage remained free-living, requiring water for fertilization, but over time, the gametophyte generation underwent a progressive reduction in size - the sporophyte phase became the dominant phase of the life cycle.

Introduction to the Pterophyta (The ferns) The ferns are an ancient lineage of plants, at least to the Devonian Period. There are approximately 11,000 species of ferns; the second largest group of plants. Ferns and other Pterophytes lacked seeds, a condition still represented by ferns (and a few other groups not discussed here). The second major lineage of land plants to evolve is referred to as the seedless, vascular plants. Even though ferns have free living gametophytes, the sporophyte is the dominant phase of the fern life cycle. Ferns produce spores (not seeds) 9. Fiddle head of new fern frond 10. Spores on the back of a fern frond

The third major period of plant evolution: The origin of the seed The seed, a specialized unit of reproduction, helped by providing a food source for the plant embryo and protecting it from harsh conditions. Seeds are also an important unit of dispersal (replacing the spore as the stage of the life cycle that disperses offspring). The seeds of gymnosperms are not enclosed in any special chambers (or ovary, as they are in the flowering plants); the name gymnosperm means, literally, “naked seed”.

Introduction to Gymnosperms The gymnosperms formed vast forests that dominated the landscape for more than 200 million years. The pine tree, a representative gymnosperm, is a sporophyte. The gametophyte generation develops from spores that are produced in male and female cones. The pollen (male gametophyte) is transferred to the ovule (female gametophyte) via wind. After fertilization, the seed begins to develop. The entire process, from cone production to seed production, can take up to three years. 11. A pine with female cones

The fourth major episode in the evolutionary history of plants: The emergence of the flower Today, the flowering plants are by far the most diverse and geographically widespread of all plants The angiosperms arose during the early Cretaceous period about 130 mya. The main feature that led to their success was the evolution of flowers and fruits. The flower is reproductive structure that bears seeds within protective chambers called ovaries. The presence of the ovary is one of the major differences between angiosperms and the gymnosperms. The ovary develops into the fruit, which is an important structure for seed dispersal. Flowers also allow for specialized pollination by attracting and rewarding pollinators. 12. A bee pollinating a flower