Seedless Plants
Introduction
Plants (kingdom Plantae) are autotrophs; they make their own organic nutrients. The term "organic" refers to compounds that contain carbon. Organic nutrients such as sugars are made by photosynthesis.
Plants are adapted to living on land. For example, the above-ground parts of most plants are covered by a waxy layer called a cuticle to prevent water loss.
Aquatic plants are secondarily adapted to living in water.
Some evidence that suggests that plants evolved from the green algae is:
they both use chlorophyll a, chlorophyll b, and carotenoid pigments during photosynthesis.
the primary food reserve of both is starch.
they both have cellulose cell walls.
These traits occur in plants
- Apical meristems
- Alternation of generations
- Spores with protective walls
- Spores produced in sporangia
- Gametes are produced in multicellular structures called gametangia; Antheridia produce sperm; Archegonia
produce eggs
- Multicellular dependent embryos
- Many have a cuticle that waterproofs and offers some protection
Alternation of Generations
The basic alternation of generations life cycle is illustrated below.
The diploid plant that produces spores is called a sporophyte. The haploid plant that produces gametes is called a gametophyte.
The evolutionary trend in plants has been from plants with a dominant gametophyte and reduced, dependent sporophyte (ex. Mosses) to plants with a dominant, independent sporophyte and a reduced, dependent gametophyte (ex. Seed plants).
Classification of Plants
Evolutionary relationships among the plants are shown below.
Bryophytes
General Characteristics
Mosses, liverworts, and hornworts lack complex vascular tissuefor transporting water and nutrients. Some mosses have simple vascular tissue.
The lack of an extensive system of vascular tissue restricts their size. The largest are less than 20 cm (8 in.).
Rhizoids are root-like structures that absorb water and nutrients. They do not have true roots, stems, or leaves because they lack vascular tissue.
Bryophytes are generally restricted to moist areas because the sperm are flagellated and therefore require at least a film of water to swim to the egg.
The sporophyte of bryophytes isdependent on the gametophyte, that is, it derives its nutrition from the gametophyte. The gametophyteisindependent.
Importance of Bryophytes
Bryophytes can colonize rocks and help initiate the soil-formation process.
Sphagnum (peat moss) is a moss that may accumulate due to low rates of decomposition. It is used for fuel in several parts of the world. It's spongy structure enables it to store water, thus making it useful for improving soil quality by adding peat moss to the soil.
Vascular Tissue
Approximately 93% of plant species are vascular plants.
Vascular plants contain vascular tissue.
There are two kinds of vascular tissue:
Xylem conducts water and minerals up from the soil. The cell walls of xylem cells help support the plant.
Phloem conducts organic nutrients from one part of the plant to another.
True roots, stems, and leaves are found only in vascular plants because these structures must contain vascular tissue.
The sporophyte of vascular plants is dominant.
Seedless Vascular Plants
General Characteristics
Seedless vascular plants include ferns, whisk ferns, club mosses, and horsetails.
The plants do not produce seeds so, like bryophytes, they are dispersed (spread) by windblown spores.
The gametophyte and sporophyte are independent.
They are vascular plants and therefore have true roots, stems, and leaves.
The sperm are flagellated and require water for reproduction. These plants are therefore limited to moist areas.
Many of the seedless vascular plants were once tree-sized. During the carboniferous period (near the end of the Paleozoic), these plants were so abundant that in some areas, their remains accumulated faster than they decomposed. These accumulations produced our fossil fuels.
Phylum Pterophyta- Ferns, Whisk Ferns, Horsetails
Ferns
In most ferns, the stem is a horizontal, underground structure called a rhizome. The leaves grow above-ground (see the photograph above).
A sorus (pl. sori) is a cluster of sporangia. Sori are located on the underside of the leaves.
/ Left: Sori ccan be seen on the underside of this fern leaf.Chapter 29 - Seed Plants
The evolutionary trend from nonvascular plants to seedless vascular plants to seed plants has been a reduction in the size of the gametophyte. In seed plants, the gametophyte is usually microscopic and is retained within the tissues of the sporophyte.
When pollen reaches the female gametophyte, it produces an elongate structure (pollen tube) that grows to the egg cell. Sperm are transferred directly through this tube to the egg. The advantage of this process is that sperm do not have to swim long distances as they do in seedless plants.
Seeds
Seeds contain the sporophyte embryo, food for the embryo, and a protective coat.
The embryo within the seed is dormant; it can survive for long periods without additional food or water. When conditions become favorable, the embryo resumes growth as the seed germinates.
Gymnosperms
The four phyla of gymnosperms are cycads, ginkgo, gnetophytes, and conifers.
Gymnosperms have naked seeds. The seeds of angiosperms are contained within a fruit.
Phylum Coniferophyta (Conifers)
Conifers are the largest group of gymnosperms. They include evergreen trees such as pine, cedar, spruce, fir, and redwood trees.
They have naked seeds produced in cones.
The leaves of conifers are needle-like and are adapted for dry conditions such as hot summers or freezing winters. Needles lose water slower than broad, flat leaves and therefore do not need to be shed during seasons when water is scarce, so most conifers are evergreen.
Conifers include the oldest and largest trees in the world. There are 4500-year-old bristlecone pines in Nevada. Redwoods in California may be greater than 90 meters tall and 2000 years old.
Pollination and Fertilization
The male gametophyte (pollen grain) consists of two cells. One small and is called a generative cell. The other, larger cell is a tube cell. The generative cell will later divide to produce two sperm.
Pollination refers to the transfer of pollen to the vicinity of the egg. The two wing-like structures on the pollen grain aid in enabling the pollen to be carried by the wind.
After being transported by wind to a seed cone, the tube cell grows toward the egg, producing a pollen tube. The two sperm produced by the generative cell enter the pollen tube and move toward the egg.
Water is not required for reproduction. During pollination, the entire male gametophyte is transferred from the pollen cone to the seed cone. The sperm are not flagellated, so they remain within the tube cell and rely on the growth of a pollen tube to deliver them to the egg cell.
Seeds
The fertilized egg (zygote) develops into an embryo which is contained within the seed.
In moss and ferns, spores were carried by the wind and functioned to disperse the species. Seeds function as a mechanism of dispersal in seed plants.
Seeds contain food and a protective coat.
Gymnosperms are plants with naked seeds (no fruit). Angiosperms (discussed below) are plants in which the seeds are enclosed within a fruit.
Angiosperms
Angiosperms are flowering plants. They are the largest group of plants with about 90% of all plant species.
They evolved from gymnosperms during the Mesozoic and became widespread during the Cenozoic.
The seeds of angiosperms are covered by a fruit. In many species, the fruit helps with dispersal of the seeds by attracting animals to consume them.
Flowers may have contributed to the enormous success of angiosperms. The flowers of many species attract animal pollinators which carry pollen to other individuals of the same species.
Diversity of Angiosperms
The oldest lineages of angiosperms are divided into three clades. The remaining lineages contain most flowering plants alive today. They are monocots, eudicots, and magnoliads.. The table below lists characteristics monocots and eudicots.
Eudicots (Dicots) / Monocotsmay be woody or herbaceous / Herbaceous
flower parts in multiples of four or five / flower parts in multiples of three
net-veined leaves / parallel-veined leaves
vascular tissue in the stem forms rings / bundles of vascular tissue are scattered throughout the stem
two cotyledons (seed leaves) / one cotyledon
Life Cycle
The life cycle of flowering plants is similar to that of gymnosperms. It involves alternation of generations. A diploid sporophyte alternates with a haploid gametophyte.
Flower
Flower parts are modified leaves. They develop within a bud.
A bud is a structure on a stem within which growth (cell division) occurs.
In many plants the same bud that previously formed leaves stops producing leaves and starts producing a flower.
Flower parts evolved as modified leaves attached to a stem tip called a receptacle.
Monocots have flower parts in multiple of threes; eudicot (dicots) parts are in multiples of fours or fives.
Below: Lily reproductive structures. These structures are described below.
Below: The stamens (anthers and filaments) and pistil (stigma, style, and ovary) have been removed from the receptacle.
Sepals
protect developing bud
Petals
The large colorful petals of many flowers function to attract pollinators.
Stamens
Stamens are composed of an anther and a filament.
Ovules
Ovules are structures that will become seeds. Contains the eggs.
Pistil
All of the female reproductive structures form the pistil. This includes the stigma, style, and ovary.
Each chamber within a pistil is called a carpel.
A simple pistil is also called a carpel because it has only one chamber.
A compound pistil contains several carpels that have become fused as a result of evolutionary change.
Ovary
The bottom portion of a pistil is the ovary. It contains ovules. As the reproductive process proceeds, the ovary enlarges and becomes the fruit and the ovules become seeds.
Below: Lily Pistil and Close-Up of Ovary
Below: Cross Section of a Lily ovary X 40
Pollination is the transfer of pollen to the stigma.
Pollen
Pollen contains two nuclei, a generative nucleus and a tube nucleus. A membrane surrounds the generative nucleus and so it is technically a cell, but it contains very little cytoplasm. The generative cell is contained within the larger tube cell.
Below: Lily pollen X 200
Pollination and Fertilization
After landing on the stigma of a flower (pollination), the tube cell elongates to produce a pollen tube, which grows from the stigma through the style and through the micropyle to the egg. The generative cell will divide by mitosis to produce two sperm. As in gymnosperms, the sperm of angiosperms are contained within the pollen tube and therefore do not require water.
Fertilization
Double Fertilization: One sperm fertilizes the egg the other one combines with the two polar nuclei forming a triploid (3N) cell.
The zygote grows by mitosis to form an embryo.
The 3N cell divides by mitosis and becomes endosperm, a food-containing material for the developing embryo.
The ovary, sometimes with other floral parts, develops into a fruit. It usually contains seeds.
Embryonic Development
In eudicots, two heart-shaped cotyledons develop and absorb endosperm, which will be used as food when the seed germinates.
Monocot cotyledons do not store endosperm. Instead, when the seed germinates, the cotyledon absorbs and transfers nutrients to the embryo.
Dormancy
Dormancy is a state in which the metabolic rate (rate of chemical reactions within the cell) slow down. The tissues within a seed become dormant, and as a result, they require very little food, oxygen, or water.
The seeds of some species can survive for many years when they are dormant. Some seeds will not germinate until after a period of dormancy.
Seed Dispersal
hooks and spines, float (coconuts), parachutes
Seeds may be dispersed when animals eat the fruits. For example, squirrels bury them for later consumption but do not always retrieve all of them.
Germination
Germination usually requires sufficient water, warmth, and oxygen.
Pollination and Coevolution
Gymnosperms are wind-pollinated.
Angiosperms are wind- or animal-pollinated.
Origin of pollination vectors
Pollinators carry plant pollen to other plants. Pollination was by wind when plants first invaded the land 400 million years ago.
Pollen is a good source of proteins and insects evolved in response to the new food supply. Plants benefited by becoming pollinated, insects benefited by receiving food.
Both species evolved to facilitate the pollination relationship. This is referred to as coevolution.
Plants evolved ability to secrete nectar, a liquid rich in sugar, proteins, and lipids. Nectar functions to attract pollinators.
Flowers attract specific pollinators
Flowers are typically shaped so that their pollinators can gain access to the nectar but other species cannot. Some examples of strategies that flowers use to attract pollinators and to limit their access to only their pollinators are discussed below.
Birds are attracted to red flowers. Bees can see colors that humans cannot. Moth-pollinated flowers are white and bloom at night.
Many insects are attracted to odors. For example, stapelia smells like rotting meat and is pollinated by flies.
Shape is important in limiting access. Flowers are often shaped to limit access. For example, hummingbird-pollinated flowers are long, and shaped like the bill of a hummingbird.
Wind Pollination
Wind-pollinated flowers are small, have no petals and little color and do not produce nectar.
Below: Many grasses are wind pollinated. The flowers are typically small and not very colorful because they do not need to attract animal pollinators.
Fruits
The ovary of flowering plants becomes the fruit. Seeds are contained within the fruit. Gymnosperms do not produce fruit.
The wall of the ovary thickens to become the pericarp of the fruit.
Fruits can be either fleshy or dry. Peaches, tomatoes, and oranges are fleshy fruits. Nuts and grains are dry fruits.
Comparisons Between Plants
Mosses have a dominant gametophyte (haploid) and a small dependent sporophyte (diploid), and are adapted to moist environments. Bryophytes and seedless plants have flagellated sperm that require water).
Ferns (seedless plants) have larger, more dominant sporophytes but still require wet conditions because of the tiny gametophyte and swimming sperm.
Where Found in Plant / Major FunctionsAuxin / Embryo of seed, young leaves, meristems of apical buds / Stimulates cell elongation; involved in phototropism, gravitropism, apical domincance, and vascular differentiation; stimualtes fruit development
Cytokinin / Synthesized in roots and transported to other organs / Stimulates cell division, involved in shoot growth, Stimulates Germination, affects root growth
Ethylene / Tissues of ripening fruits, nodes of stems, senescent leaves and flowers / Stimulates fruit ripening, inhibits leaf and flower growth
Abscisic Acid / Leaves, stems, green fruit / Stimulates stomata closure during stress
Gibberellin / Meristems of apical buds and roots, young leaves, embryo / Stimulates shoot elongation, stimulates flowering, and fruit development, promotes seed germination
Gymnosperms and angiosperms are widespread and well adapted to land because of their large sporophytes and the coverings of the spores, gametophytes, gametes, zygotes, and embryos. Their sperm do not require water.
Plants move in response to both internal and external signals known as stimuli. Plant movements fall into two categories: tropisms and nastic movements.
Tropisms are responses in which the direction of the movement is determined by the direction of the stimuli. Tropisms are growth movements that happen slowly and whose results are irreversible.
If a plant reacts toward the stimuli, this is said to be a positive tropism but if the plant reacts away from the stimuli, this is termed a negative tropism. Plants respond in this fashion to the external forces of gravity, contact (touch), direction of light, water, and fluctuation in temperature.
Geotropism or Gravitotropism is Growth Influenced by Gravity
The effect of gravity upon the direction of growth is termed geotropism (or gravitropism) In plants, two main response exist to gravity: negative geotropism, as in the upward growth of the shoot of a plant, and positive geotropism, as in the downward growth of the root of a plant.
Growth of plant parts towards or away from gravity is controlled by auxin. Higher or lower concentrations of auxins in various places in roots and stems stimulate growth toward or away from gravity.
Thigmotropism is Growth Influenced by Contact
Thigmotropism is the term applied to growth movements made by plants in response to contact with a solid object. Most such movements are curvatures and they are most impressive in the tendrils of climbing plants. A tendril is a thread – like growth that may be a modified leaf or part of a leaf, or a modified branch. Certain plant families, such as the pea, grape, and gourd, are outstanding in their climbing habits.
The curvature of the tendril that follow contact with a support are the result of increased growth on the side opposite the stimulus. Contact with a solid object causes the auxin to migrate to the side away from the contact. The cells on the far side of the contact lengthen more than those on the contact side and thus the tendril grows around the contact object. In the gourd family (pumpkin, squash, cucumber) a complete coil may form around a support within 1 to 10 minutes after contact, depending on the age of the tendril.