What are land plants?

• A land plant is a multicellular eukaryote that:

o Obtains carbon from CO2 and energy from light.

o Has cells each enclosed by a wall made mainly of cellulose.

o Has embryonic development.

o An embryo is an early developmental stage that is protected within the shell, seed, or other parental tissues of an organism.

o Has a haploid-diploid sexual life cycle.

Clockwise from top left: Moss, fern, conifer, flowering plant.

Clade Living Members

Nonvascular Plants

Liverworts Hornworts Mosses

Seedless Vascular Plants

Club mosses Ferns, horsetails, whisk ferns

Gymnosperms

Ginkgos Cycads Gnetophytes Conifers

Angiosperms Flowering Plants

• Land plants can be grouped into four clades based on characteristics such as the presence or absence of vascular tissue, seeds, and flowers.

o Vascular tissue (xylem and phloem) transports water and nutrients throughout the plant.

Cross section of celery stalk showing vascular bundles, each of which contains both phloem and xylem.

The Ancestor of Land Plants

• Green algae called charophytes (or stoneworts) are the closest living relatives of land plants. Evidence for this includes:

o Both have chloroplasts.

o Both have cell walls made of cellulose.

o Both store sugar as starch.

o Both use chlorophyll a and b for photosynthesis.

o DNA sequence data supports a close relationship.

Chara, a green alga. The gametophyte is the dominant life stage.

• In charophytes, haploid adults (gametophytes) produce sperm that swim to the eggs. Immediately after fertilization, the diploid zygote undergoes meiosis.

o This called a haploid life cycle.

o Instead of gametes, meiosis produces four haploid cells called spores.

o Each spore can develop into another gametophyte.

The Ancestral Algal Life Cycle

• The transition to land by the ancestor of plants carried both promise and peril.

o The risk was a harsh environment:

o Temperature extremes

o Desiccation (drying out)

o Lack of water to permit fertilization

o Damage by UV radiation

o Lack of structural support against gravity

o The rewards were great opportunities:

o More CO2 (which is more concentrated in air than in water)

o More light (unfiltered by water)

o A more reliable supply of minerals and other nutrients (which are more concentrated in soil than at the surface of a lake or sea)

o No or few competitors or predators (until insects arrived)

• Biologists think that ancestral charophytes lived in shallow water that sometimes dried out (as do modern charophytes).

• Natural selection would have favored adaptations in these charophytes to resist drying out, such as a waxy cuticle and layers of tissue in the parent to protect developing embryos. These traits would have then assisted the transition onto land.

• The fundamental difference between land plants and charophytes is the growth of an embryo within the tissues of the parent plant.

o Because this difference is so basic, land plants are sometimes referred to as "embryophytes".

Here, embryos will develop within the tissue of a modern moss .

• Land plants keep the gametophyte, but the zygote divides by mitosis (not by meiosis), producing an embryo that develops into a diploid adult (sporophyte).

o This is called a haploid-diploid life cycle.

o Compared to a zygote, a sporophyte is a "spore factory".

o A huge number of spores is a useful adaptation for life on land, where fertilization is difficult.

The Land Plant Life Cycle

• The land plant life cycle is called "Alternation of Generations" because two multicellular body forms alternate: the sporophyte and the gametophyte.

Alternation of Generations

Nonvascular Plants

• The first land plants to evolve were nonvascular. The most common of the nonvascular plants today are the mosses.

o Their life cycle is dominated by the gametophyte.

o In mosses, the gametophyte plant produces both sperm and eggs.

o As in their algal ancestors, the sperm must still swim through water to reach the egg, so nonvascular plants must live in moist environments where water is likely to be on the surface of the gametophyte.

o The embryo sporophyte remains on its gametophyte parent and develops into a smaller, non-photosynthetic plant that lives as a parasite on the gametophyte.

o The sporophyte releases airborne spores.

• Because nonvascular plants lack xylem and phloem, they also lack "true" leaves, stems, and roots.

o As a result, they live low to the ground and absorb water and nutrients from their surroundings through osmosis.

Seedless Vascular Plants

• Most of the seedless vascular plants that flourished in the past are extinct. The most common today are the ferns. They differ from nonvascular plants in that their sporophytes:

o Dominate the life cycle.

o Are photosynthetic and do not live on the gametophyte.

o Have xylem and phloem.

o Have "true" roots, stems, and leaves.

Vascular tissue is a major plant adaptation to living on land.

• As in nonvascular plants, when the young fern sporophyte emerges, it is essentially a parasite on the gametophyte. But when the sporophyte grows large enough to produce its own food, it becomes independent from its gametophyte parent. The vaguely heart-shaped green structure

to the left is the fern gametophyte. A single leaf from a newly-formed sporophyte is emerging from the lower surface of the

gametophyte.

• Although the sporophytes of seedless vascular plants are better adapted to life on land, their fragile gametophytes are not because they:

o Lack vascular tissues.

o Still require the medium of water for fertilization.

o Are prone to desiccation, being trampled, being eaten, and being out shaded by the surrounding foliage.

Notice how small this mature fern gametophyte is – barely half a

centimeter long.

How did seeds evolve?

• The seed began to evolve when the sporophytes of some early vascular plant species began retaining rather than releasing their spores.

o The tissues of the parent sporophyte protected the spores and developing gametophytes from environmental stresses, such as UV radiation and desiccation.

o The non-photosynthetic, microscopic gametophytes obtain nutrients from the parent sporophyte.

o The relationship between the gametophyte and the sporophyte in seed plants is thus the reverse of that in nonvascular plants.

• The early spore-retaining vascular plants also evolved two types of spores.

o Most seedless plants make only one type of spore, which grows into a gametophyte that can produce both sperm and eggs — and sperm must swim to the egg.

o However, each spore of the early spore-retaining vascular plants was either male or female, growing into either a male or female gametophyte.

o The male gametophytes produce sperm.

o The female gametophytes produce eggs.

• The male gametophyte further evolved into the form of pollen grains.

o Pollen grains (male gametophytes) are released from the sporophyte and carried by wind or animals to the part of the seed plant that contains the female gametophyte.

o This transfer of pollen to the female gametophyte is called pollination.

o Pollination eliminates the need for water through which sperm swim to the egg.

Pollen and Pollination

Pollen grain of the Eastern White Pine (Pinus strobes), 2290x.

Pollen grains of various species of seed plants.

• When the pollen grain arrives near the female gametophyte, it produces two sperm cells and a pollen tube.

o The female gametophyte and the surrounding layer(s) of protective sporophyte tissue are together called an ovule.

o The pollen tube penetrates the ovule, carrying two sperm cells to the female gametophyte.

Pollen tube of pine

pollen grain

• Following fertilization, the ovule develops into a seed, which can disperse the embryo away from the parent sporophyte.

o The outer layer of the ovule made by the parent sporophyte becomes the seed coat, which protects the embryo.

o Seeds also let the embryo remain dormant until conditions are right to resume growth (that is, germinate).

o Dormancy is a state in which the metabolic rate (rate of chemical reactions within an organism) slows down.

o As a result, the dormant embryo within a seed requires very little food, oxygen, or water.

o The seed also contains food for the embryo when it germinates.

The Seed

• Both seeds and spores can disperse offspring by wind to new locations, but seeds provide several advantages over spores:

o Spores are usually single-celled whereas seeds are multicellular, consisting of an embryo protected by the seed coat.

o A seed can remain dormant for days, months, or even years after being released from the parent plant, whereas most spores have a shorter lifetime.

o Unlike spores, seeds have a supply of stored food.

Gymnosperms

• The first seed plants were gymnosperms, which have neither flowers nor fruit.

o The most common gymnosperms today are the conifers.

o The male and female gametophytes of gymnosperms are located on separate cones.

o The pollen and seeds of gymnosperms are dispersed by wind but not by animals.

o One of the sperm cells produced by a pollen grain fertilizes an egg and the other dies.

o The female gametophyte within the ovule becomes a food reserve for the embryo within the seed (even if there is no embryo!).

Pollen being released by a pine tree.

Pollen Being Released by Pine Trees https://www.youtube.com/watch?v=n9E5ctLoi0Q

Pine seed, cross-section. The well-developed embryo, food reserve, and a bit of the surrounding seed coat are visible.

Food Reserve (former female gametophyte)

Embryo Sporophyte

Seed Coat (from ovule)

• Angiosperms evolved from a gymnosperm ancestor and are by far the most common land plants today.

o Male and female gametophytes are located in separate structures within flowers.

o The ovule is contained within the ovary, the wall of which develops into a fruit that contains the seeds.

o Because of the flower and fruit, pollen and seeds are dispersed much more often by animals than by wind.

o One of the sperm cells from a pollen grain fertilizes an egg cell and the other fertilizes a "central cell" in the female gametophyte.

o The fertilized central cell forms a food reserve called endosperm for the embryo within the seed.

Angiosperms

• The four basic parts of a flower are arranged concentrically. They are (from outer to inner):

o Sepals. Leaf-like parts that enclose and protect the inner parts of the flower in the bud stage.

o Petals. Leaf-like parts that also enclose and protect the inner parts.

o They are often colored to attract pollinators such as insects and birds.

o Stamens. Each stamen consists of a slender, flexible filament which supports the anther at its end.

o Anthers contain developing male gametophytes (pollen grains).

o One or more carpels. Each carpel consists of a lower, swollen portion called the ovary and a long and slender style that supports the stigma at the top of the carpel.

o Ovaries contain ovules that each contain a developing female gametophyte.

Cross-section of Lilium flower, with 6 stamens and 3 carpels.

• The flower is an adaptation to attract animals to transport pollen to the stigma of a carpel, using lures such as:

o Visual cues: color and shape

o Fragrance

o Food: nectar or even pollen itself

o Chemicals: narcotics and sexual attractants (pheromones)

A Bee orchid has evolved over many generations to better mimic a female bee to

attract male bees as pollinators.

• Pollination using animals, including insects, bats, birds, and small rodents, is much more efficient than relying on the wind, as in gymnosperms.

o When was the last time you saw clouds of pollen being released by a flower?

Double Fertilization

• At the stigma, the pollen grain produces a pollen tube that carries two sperm to the female gametophyte.

o One sperm cell fertilizes the egg cell, forming the zygote.

o The other sperm cell fertilizes the central cell, which eventually forms the endosperm.

• The fertilization of two cells in the female gametophyte is called double fertilization and occurs only in angiosperms.

o The endosperm is a much more nutritious food reserve than that stored within the gymnosperm seed.

o Double fertilization may have evolved as a way of preventing flowers from wasting resources by producing a food reserve in ovules that lack fertilized eggs.

Monocots and Dicots

• There are two kinds (not clades) of angiosperms based on the number of cotyledons in its seeds.

o Cotyledons are leaf-like structures where food is stored for the germinating embryo.

o The food in the cotyledons is derived from the endosperm.

o A dicot (aka eudicot) has two cotyledons.

o A monocot has only one.

Examples of Monocots

Examples of Dicots

Tulips Magnolia trees

Orchids Sunflowers

Daffodils Geraniums

Bamboo Soybean plants

Palms Pea plants

Wheat Oaks

Corn Maples

Fruits

• The fruit is an adaptation that promotes dispersal of seeds, often by animals, using techniques such as:

o Edible fruits. They entice animals to eat them but the seeds pass through their gut unharmed and may end up some distance away from the parent plant.

o Water. Floating fruits are carried to new locations by water currents.

o Hitchhikers. They stick to the coat of a passing animal.

o Wind. They are light enough to be airborne, such as those of maples and dandelions.