Overview of Plants

The Evolution of Plants All plants are multicellular

autotrophs that make food by photosynthesis. All plants contain chlorophyll.

Most plants are terrestrial. They and fungi both descended from multicellular protists.

Aquatic life was protected from cosmic radiation by the water they lived in. This was not true of the organisms that began to move out onto the land.

Plants and fungi first colonized the land about the same time, about 440 m.y.a. when atmospheric O2 and O3 (ozone) accumulated in large enough amounts to protect the living organisms from cosmic radiation.

Before plants could survive on land, they had to solve three serious problems:

1) They had to be able to absorb needed minerals from the rocky surface of the land.

2) They had to prevent water loss in the dry environment and support their bodies.

3) They had to have a way to reproduce on land.

Problems Solved! Absorbing Minerals: The first land plants did

not have roots. Symbiotic relationships called mycorrhizae developed between the first land plants and early fungi. The plants make carbohydrates needed by both organisms by photosynthesis, and the fungi absorb from the rocky land the minerals needed by both organisms.

Conserving Water and Supporting the Plant Body: The first land plants lived by the edges of bodies of water so they could replace lost water. As plants developed a watertight waxy cuticle, they could then move into drier environments. The plant cuticle is waterproof, but it also is airtight. Plants developed specialized holes in the cuticle called stomata for gas exchange. Guard cells open and close the stomata to exchange gases while preventing water loss.

Plants do not have skeletons like animals do. Instead, individual plant cells have stiff cell walls made of cellulose for support. They also use water pressure in the cell’s central vacuole to help plant cells retain their shape. Large plants have developed special woody layers that also contribute to supporting the plant body in air.

Reproducing on land: The gametes of land plants must be able to move from the male to the female plant without drying out. Primitive plants still need water to reproduce, but higher plants surround sperm in multi-layered structures called pollen grains, which are transmitted by wind or animals, rather than water.

Evolution of a Vascular System The first land plants did not look

different above or below the ground. As later land plants evolved, they developed specialized structures such as stems, roots, and leaves to help them adapt to their new environment.

A vascular system transports materials like water and food throughout the plant body. Early plants also lacked a vascular system. All materials had to be transported by osmosis or diffusion. This limited plant size. All non-vascular plants are very small.

Mosses have very simple vascular tissue, but it is not well-developed. They do not have a vascular system.

The first vascular plants appeared 430 m.y.a. Today, vascular plants dominate almost every habitat.

Modern vascular plants can grow very tall, and have 3 features in common :

1) All vascular plants have a dominant sporophyte form in their life cycle.

2) All vascular plants have specialized conducting tissues called xylem (transports water) and phloem (transports food).

3) The bodies of vascular plants have a central shaft from which specialized structures branch out. The roots (below ground) are different from the shoots (above ground).

Alternation of Generations

The sporophyte generation is diploid and undergoes meiosis to produce haploid spores, which grow up into the gametophyte.

The gametophyte generation is haploid and produces eggs or sperm by mitosis. The eggs and sperm unite during fertilization to form the zygote, which grows into the diploid sporophyte and completes the cycle.

Plants exist in two life forms that alternate with each other: the sporophyte generation and the gametophyte generation.

Sporophyte (diploid)

Gametophyte (haploid)

(Meiosis)

(Growth by Mitosis)

eggs

sperm

(Mitosis)

Fertilization ! (diploid zygote)

Haploid Spores

Evolution of Plant Life Cycles The life cycle of non-vascular plants is

dominated by a large gametophyte generation that supports a smaller, dependent sporophyte generation.

The fern life cycle represents an intermediate stage in the evolution of plant life cycles.

In ferns and other seedless vascular plants, the sporophyte is dominant and the gametophyte is smaller, but independent and self-sufficient.

In seed plants, the gametophyte has become much smaller, and are entirely dependent on the sporophyte generation for all the necessities of life.

The seed plant you see is the sporophyte generation. Gametophytes produce either eggs or sperm, which are haploid.

Seedless vascular plants (ferns)

Dominant Sporophyte

Smaller, but Independent Gametophyte

Non-vascular plants (mosses and liverworts)

Large, Dominant Gametophyte

Small, Dependent Sporophyte

Higher vascular plants

Large and Dominant

SporophyteSmall, Dependent

Gametophyte

Evolution of Seeds – Part 1: Ferns Following the development

of a vascular system, the next great advance was the development of seeds.

The first vascular plants did not produce seeds.

Ferns reproduce using spores.The great carboniferous fern forests of the Paleozoic Era needed a moist climate. Like non-vascular plants, ferns have swimming sperm and need some water for fertilization to occur.

The fern life cycle represents an intermediate stage between the lower plants and the seed plants. The sporophyte stage is dominant, but the gametophyte, while smaller, is still able to live independently.

The fern sporophyte consists of roots, underground stems called rhizomes, and long, highly divided leaves called fronds.

The fern gametophyte is a small thin, heart-shaped photosynthetic plant that lives in moist places and is no more than 1 cm in diameter.

The fern gametophyte produces eggs in organs called archegonia and sperm in organs called antheridia. The same plant has both male and female sex organs. When a film of water is present, sperm are able to swim to the eggs and fertilize them.

Evolution of Seeds – Part 2: Gymnosperms

The word gymnosperm means “naked seed.” Gymnosperm seeds do not develop inside of a fruit (a mature ovary).

Gymnosperms first appeared about 380 m.y.a., and were the first seed plants. The flowering plants (angiosperms) evolved from gymnosperms between 150-200 m.y.a. and are the most recently evolved plant phylum.

In seed plants, the gametophyte is very much reduced in size, and is no longer independent. Seed plants produce two kinds of gametophytes: the male (microgametophyte) and the female (megagametophyte).

The sporophyte generation produces two kinds of spores: microspores, which produce the male gametophyte, and megaspores, which produce the female gametophyte.

Pollen grains are actually microspores, that produce the male gametophyte and its sperm.

An ovule contains a megaspore that produces the female gametophyte and its egg.

In many gymnosperms, the fertilized egg (zygote) is retained in a cone until it is mature.

Pollen grain (microspore) Ovule (megaspore)

Male gametophyte

sperm

Female gametophyte

egg

What is a Seed? A seed is a sporophyte plant embryo

surrounded by a protective seed coat, which keeps the embryo from drying out. In addition, seeds often contain a food supply for the developing embryo.

Seeds have allowed plants to become adapted to life on land in three major ways:

1) Dispersal — many seeds have appendages that aid in dispersal by water, wind, or animals. Dispersal prevents competition between parent and offspring.

2) Nourishment — Most kinds of seeds have lots of food stored within them, which provides the young embryo with an energy source to begin its growth.

3) Dormancy — Seeds can lie dormant for many years, protecting the embryo from unfavorable conditions

Evolution of Flowers Gymnosperms have not efficiently solved

the problem of fertilization. Many pollen grains must be carried by wind in order to ensure the joining of sperm and egg within a female cone.

Angiosperms often have pollen delivered directly from one individual to another, thus greatly increasing the chance of fertilization. This innovation is made possible by the evolution of the flower.

What is a flower? It is the reproductive structure of angiosperms. The basic structure of a flower consists of four concentric whorls of appendages:

Calyx — outermost whorl, made of sepals, which are modified leaves, and protect the bud from damage.

Corolla — next layer, made of one or more petals, which are also modified leaves, but produce bright pigments or strong fragrances to attract animals.

Androecium — produces the microgametophytes, or pollen grains. This layer is made of the stamens.

Gynoecium — is the innermost layer of the flower and consists of one or more pistils, containing the ovary and ovules.

Flowers containing all four parts are complete, while flowers missing one or more parts are incomplete.

Layers of a Flower

Parts of a Flower

The inside layer of a flower contains the female reproductive structures. These may be one or more pistils. The pistil is subdivided into the ovary, where ovules develop, the sticky stigma, where pollen lands, and the style which connects them to each other.

The next layer of a flower contains the male reproductive structures. Stamens consist of two parts: the filament, a strong stalk, and the anther, which rests on the filament and produces pollen grains.

The outermost layer of a flower consists of sepals, which serve to protect the developing flower parts. Inside of this is the whorl of petals, which are often brightly colored to attract animals.

Petals

Sepals

Stamen:

Anther

Filament

Pistil:

Stigma

Style

Ovary with ovules