How Plants Colonized Land Plant Diversity 1 How Plants Colonized Land

From Water to Land In order to survive the transition from water to land, it was necessary for plants to make adaptations for obtaining water and to prevent them from drying out.

Reproduction in Water Plants Algae reproduce with flagellated sperm cells which require water for fertilization. Land plants must be able to reproduce without water.

Surviving UV radiation Once plants emerge from the protective cover of water, the genetic material (DNA) is more susceptible to damage by ultraviolet radiation.

Major Plant Adaptations for Survival on Land: 1. Except for primitive bryophytes, the dominant generation of all plants is the diploid sporophyte generation. A diploid structure is more apt to survive genetic damage because two copies of each chromosome allows recessive mutations to be masked.

Major Plant Adaptations for Survival on Land: 2. All plants possess a cuticle, a waxy covering on aerial parts that reduces desiccation (drying out).

Major Plant Adaptations for Survival on Land: 3. The development of a vascular system in plants further reduced their dependency on water. Without a vascular system, all cells must be reasonably close to water. A vascular system allows for water to be distributed throughout the plant.

Major Plant Adaptations for Survival on Land: 4. In the more primitive plants, flagellated sperm require water to swim to the eggs. In the more advanced plants, the sperm, packaged as pollen, are adapted for delivery by wind or animals.

Major Plant Adaptations for Survival on Land: 5. In the most advanced division of plants, the Anthophyta*, the gametophytes are enclosed (and thus protected) inside an ovary. *These are the flowering plants.

Major Plant Adaptations for Survival on Land: 6. The most advanced plants (Conifers and Flowering plants) have developed adaptations to seasonal variations in the availability of water and light. Ex: Deciduous Trees shed their leaves to minimize water loss during the dormant season. Desert wildflowers germinate, grow, bloom and produce seeds during the brief time there is water after a rain.

Key Idea! Understand how the various plant types evolved: Chlorophytes → Bryophytes → Seedless Vascular Plants → Gymnosperms → Angiosperms

Chlorophyta—Green Algae Scientists have found enough evidence to conclude that they are the common ancestors of land plants. They have both chlorophyll a and b. They have cellulose cell walls and store their carbohydrates as starch. Some species show trends towards multicellular colonies. Microasterius Cosmarium Volvox colony Desmidium

Bryophytes Bryophytes are considered the most primitive land plants. They were the first to evolve from the chlorophytes. They include mosses, liverworts, and hornworts.

Bryophytes’ Evolution from Water: In order to survive on land where water is not unlimited, two evolutionary adaptations helped: 1) A waxy cuticle to protect against water loss 2) the packaging of gametes in structures known as gametangia. The gametangia provide a safe haven because the fertilization and development of the zygote occur within the protected structure.

Alternation of Generations in Plants: In plants, a multicellular sporophyte (diploid, 2N) alternates with a multicellular gametophyte (haploid, 1N) In plants, meiosis produces spores. Spores are haploid cells that divide by mitosis to become a multicellular haploid organism, the gametophyte. Gametes are produced by the gametophyte by mitosis since it is already haploid.

Alternation of Generations in Plants: When both the gametophyte and sporophyte stages are multicellular, the life cycle demonstrates an alternation of generations. The gametes fuse and produce a diploid cell that grows by mitosis to become the sporophyte. Specialized cells in the sporophyte divide by meiosis to produce spores.

Bryophyte Reproduction In bryophytes, the gametes are packaged in structures called gametangia. Bryophyte sperm is produced by the male gametangia, called antheridia. Bryophyte eggs are produced by the female gametangia, called archegonium. The gametangia provide a safe haven because the fertilization and development of the zygote occur within the protected structure.

Liverworts Liverworts are very small plants (usually less than 1” in size) Like all Bryophytes, they live in very moist places since they reproduce with flagellated sperm that must swim through water to fertilize the eggs.

Liverworts Like other bryophytes, liverworts do not have vascular tissues to carry water. This lack of vascular tissue (plus the fact they have flagellated sperm cells) results in a dependence on water. For this reason, bryophytes must live in damp areas so they don’t dry out.

Hornworts Reproductive structures at the tips release spores

Hornworts The group's common name "hornwort" refers to the tall narrow sporophytes which are embedded in the top of the plant.

Mosses Mosses are special because, unlike in all other plants, the dominant generation in their life cycle is the haploid gameophyte. (What you see when you look at a moss plant is a collection of haploid cells!)

Mosses The moss sporophyte (2N) is tiny, short-lived, and relies on the gametophyte (1N) for nutritional support.

Tracheophytes—The First Vascular Plants The transition from water to land was tricky—how to get nutrients and water from the soil up to all parts of the plant? The answer: Phloem and Xylem—Vascular tissues that transport materials.

Vascular Plants Xylem—water superhighway, transporting water from the soil throughout all parts of the plant Phloem—Sugar food superhighway, transporting sugar and nutrients.

Tracheophytes—First Vascular Plants The first vascular plants to evolve did not have seeds. They live their lives as sporophytes (2N). They produce spores (haploid) which are bisexual (neither male nor female)

Ferns-Spores on Leaves The spores of ferns live on the underside of the leaves. The spores will germinate to form tiny gametophytes under the leaf surface.

Fern Life Cycle http://academic.kellogg.edu/herbrandsonc/bio111/animations/0124.swf

Seed Plants Seeds are packages containing an embryo and the food to feed the developing embryo that is surrounded by a nice protective shell.

Evolution of Seed Plants 3 major changes led to the evolution of seed plants: 1. Further decline in the prominence of the gametophyte generation of the life cycle. 2. The birth of pollination 3. The evolution of the seed.

Gymnosperms The first major seed plants to surface were the gymnosperms (“naked” “seeds”) The most important gymnosperms are the Conifers. Their reproductive structures are cones. They include pine trees, firs, cedars and redwoods.

Gymnosperms Gymnosperms produce both male and female gametes. Gymnosperms transport their sperm through the use of pollen. These plants survive well in dry conditions and keep their leaves all year. They usually have needles for leaves.

Pollen Pollen is the sperm-bearing male gametophyte (1N). Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the female cone of conifers.

Gymnosperms Life Cycle

Flowering Plants The final major plant evolutionary category to branch off are the flowering plants. Today, there are more of these than any other plants.

Angiosperms Flowers are the reproductive structures of the angiosperms (“covered” “seeds”)

Angiosperms The flower is a major evolutionary advancement because: It attracts pollinators such as insects and birds The ovules are protected inside the ovary The ovary develops into a fruit which fosters the dispersal of seeds by wind, insects, birds, mammals and other animals.

Angiosperm Life Cycle

Major Evolutionary Trends in the Plant Kingdom: Dominant gametophyte generation → dominant sporophyte generation Nonvascular → vascular Seedless → seeds Motile sperm → pollen Naked seeds → seeds in flowers