26: Seed Plants Flashcards
Evolution of Seed Plants, Gymnosperms, Angiosperms, the Role of Seed Plants
What is a flower?
Branches specialized for reproduction found in some seed-bearing plants, containing either specialized male or female organs or both male and female organs.
What is fruit?
Thickened tissue derived from ovary wall that protects the embryo after fertilization and facilitates seed dispersal.
What is an ovule?
A female gametophyte.
What is a pollen grain?
A structure containing the male gametophyte of the plant.
What is a pollen tube?
An extension from the pollen grain that delivers sperm to the egg cell.
What is a progymnosperm?
A transitional group of plants that resembled conifers because they produced wood, yet still reproduced like ferns.
What is a seed?
A structure containing the embryo, storage tissue and protective coat.
What is a spermatophyte?
A seed plant; from the Greek sperm (seed) and phyte (plant).
What are some examples of the uses of seed plants?
For millennia, human societies have depended on seed plants for nutrition and medicinal compounds: and more recently, for industrial by-products, such as timber and paper, dyes, and textiles. Palms provide materials including rattans, oils, and dates. Wheat is grown to feed both human and animal populations. The fruit of the cotton boll flower is harvested as a boll, with its fibers transformed into clothing or pulp for paper. The opium poppy is valued both as an ornamental flower and as a source of potent opiate compounds.
What are the sizes of the sporophytes and gametophytes in seed plants?
In seed plants, the evolutionary trend led to a dominant sporophyte generation, and at the same time, a systematic reduction in the size of the gametophyte: from a conspicuous structure to a microscopic cluster of cells enclosed in the tissues of the sporophyte.
Are seed plants homosporous or heterosporous?
Whereas lower vascular plants, such as club mosses and ferns, are mostly homosporous, all spermatophytes are heterosporous.
What are the two types of spores produced by seed plants?
Spermatophytes produce two types of spores: megaspores (female) and microspores (male). Megaspores develop into female gametophytes that produce eggs, and microspores mature into male gametophytes that generate sperm.
Are the gametophytes of seed plants free-living?
Because the gametophytes mature within the spores, they are not free-living, as are the gametophytes of seedless vascular plants.
What are the evolutionary forerunners of seed plants?
Heterosporous seedless plants are seen as the evolutionary forerunners of seed plants.
Which adaptations distinguish seed plants from other vascular plants?
Seeds and pollen - two critical adaptations to drought, and to reproduction that doesn’t require water - distinguish seed plants from seedless vascular plants. Both adaptations were required for the colonization of land begun by the bryophytes and their ancestors.
When did gymnosperms evolve?
The first reliable record of gymnosperms dates their appearance to the Pennsylvanian period, about 319 million years ago.
What was the evolutionary success of gymnosperms versus angiosperms?
Gymnosperms dominated the landscape in the early (Triassic) and middle (Jurassic) Mesozoic era. Angiosperms surpassed gymnosperms by the middle of the Cretaceous (about 100 million years ago) in the late Mesozoic era, and today are the most abundant plant group in most terrestrial biomes.
What are the names of the eons and when did they occur?
- Phanerozoic, from 541 million years ago to present
- Proterozoic, from 2.5 billion years ago to 541 million years ago
- Archean, from 3.8 to 2.5 billion years ago
- Pre-Archean, from the formation of the Earth to 3.8 billion years ago
What are the names of the eras of the Phanerozoic eon and when did they occur?
- Cenozoic, from 66 million years ago to present
- Mesozoic, from 252 million years ago to 66 million years ago
- Paleozoic, from 541 million years ago to 252 million years ago
What are the names of the periods of the Cenozoic era and when did they occur?
- Quaternary, from 2.6 million years ago to present
- Tertiary, from 66 million years ago to 2.6 million years ago
What are the names of the periods of the Mesozoic era and when did they occur?
- Cretaceous, from 145 mya to 66 mya
- Jurassic, from 201 mya to 145 mya
- Triassic, from 252 mya to 201 mya
What are the names of the periods of the Paleozoic era and when did they occur?
- Permian, from 298 to 252 mya
- Pennsylvanian, from 323 to 298 mya
- Mississippian, from 358 to 323 mya
- Devonian, from 419 to 358 mya
- Silurian, from 443 to 419 mya
- Ordovician, from 485 to 443 mya
- Cambrian, from 542 to 485 mya
How do seeds facilitate reproduction in seed plants?
The small haploid cells are encased in a protective coat that prevent desiccation and mechanical damage. Pollen grains can travel far from their original sporophyte, spreading the plant’s genes. The seed offers the embryo protection, nourishment, and a mechanism to maintain dormancy for tens or even thousands of years, ensuring germination can occur when growth conditions are optimal. Seeds therefore allow plants to disperse the next generation through both space and time.
What is the earliest seed plant discovered?
The fossil plant Elkinsia polymorpha, a “seed fern” from the Devonian period - about 400 million years ago - is considered the earliest seed plant known to date.
What are some characteristics of the first true seed plants and when did they appear?
Seed ferns produced their seeds along their branches without specialized structures. What makes them the first true seed plants is that they developed structures called cupules to enclose and protect the ovule - the female gametophyte and associated tissues - which develops into a seed upon fertilization. Seed plants resembling modern tree ferns became more numerous and diverse in the coal swamps of the Carboniferous period.
When did progymnosperms appear?
Fossil records indicate that the first gymnosperms (progymnosperms) most likely originated in the Paleozoic era, during the middle Devonian period, about 380-390 million years ago.
Why were seed plants successful during the Permian period?
Following the wet Mississippian and Pennsylvanian periods, which were dominated by giant fern trees, the Permian period was dry. This gave a reproductive edge to seed plants, which are better adapted to survive dry spells.
What were the first gymnosperms to appear during the lower Jurassic?
The Ginkgoales, a group of gymnosperms with only one surviving species – the Ginkgo biloba.
When did gymnosperms reach their evolutionary peak?
Gymnosperms expanded in the Mesozoic era (about 240 million years ago), supplanting ferns in the landscape, and reaching their greatest diversity during this time. The Jurassic period was as much the age of the cycads (palm-tree-like gymnosperms) as the age of the dinosaur. Ginkgoales and conifers also dotted the landscape.
What ecosystems do gymnosperms dominate nowadays?
Although angiosperms are the major form of plant life in most biomes, gymnosperms still dominate some ecosystems, such as the taiga (boreal forests) and the alpine forests at higher mountain elevations because of their adaptation to cold and dry growth conditions.
How are seeds an evolutionary adaptation to dry land?
Unlike bryophyte and fern spores (which are haploid cells dependent on moisture for rapid development of gametophytes), seeds contain a diploid embryo that will germinate into a sporophyte. Storage tissue to sustain growth and a protective coat give seeds their superior evolutionary advantage. Several layers of hardened tissue prevent desiccation, and free reproduction from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy - induced by desiccation and the hormone abscisic acid - until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.
How do pollen grains facilitate sexual reproduction?
Pollen grains are male gametophytes and are carried by wind, water, or a pollinator. The whole structure is protected from desiccation and can reach the female organs without dependence on water. Male gametes reach the female gametophyte and the egg cell gamete through a pollen tube: an extension of a cell within the pollen grain.
Do the male gametes of gymnosperms have flagella?
The sperm of most modern gymnosperms lack flagella, but in cycads and the Ginkgo, the sperm still possess flagella that allow them to swim down the pollen tube to the female gamete; however, they are enclosed in a pollen grain.
When did angiosperms appear?
Undisputed fossil records place the massive appearance and diversification of angiosperms in the middle to late Mesozoic era. Fossil evidence indicates that flowering plants first appeared in the Lower Cretaceous, about 125 million years ago, and were rapidly diversifying by the Middle Cretaceous, about 100 million years ago. Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, including pollinating insects that played a key role in ecology and the evolution of flowering plants.
How did angiosperms evolve?
Although several hypotheses have been offered to explain the sudden profusion and variety of flowering plants, none have garnered the consensus of paleobotanists. New data in comparative genomics and paleobotany have, however, shed some light on the evolution of angiosperms. Rather than being derived from gymnosperms, angiosperms form a sister clade that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, which increased genetic variability and range. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were basal angiosperms related to tropical grasses. Both views draw support from cladistics studies, and the so-called woody magnoliid hypothesis - which proposes that the early ancestors of angiosperms were shrubs - also offers molecular biological evidence.
What is the most primitive living angiosperm?
The most primitive living angiosperm is considered to be Amborella trichopoda, a small plant native to the rainforest of New Caledonia, an island in the South Pacific. Analysis of the genome of A. trichopoda has shown that it is related to all existing flowering plants and belongs to the oldest confirmed branch of the angiosperm family tree.
Why are basal angiosperms considered primitive?
A few angiosperm groups called basal angiosperms are viewed as primitive because they branched off early from the phylogenetic tree. Most modern angiosperms are classified as either monocots or eudicots, based on the structure of their leaves and embryos. Basal angiosperms, such as water lilies, are considered more primitive because they share morphological traits with both monocots and eudicots.
Where does fertilization and embryo development occur in angiosperms?
Angiosperms produce their gametes in separate organs, which are usually housed in a flower. Both fertilization and embryo development take place inside an anatomical structure that provides a stable system of sexual reproduction largely sheltered from environmental fluctuations.
How diverse are flowering plants?
Flowering plants are the most diverse phylum on Earth after insects; flowers come in a vast array of sizes, shapes, colors, smells, and arrangements.
What is the relationship between flowers and pollinators?
Most flowers have a mutualistic pollinator, with the distinctive features of flowers reflecting the nature of the pollination agent. The relationship between pollinator and flower characteristics is a primary example of coevolution.
How do seeds and fruits develop?
Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Not all fruits develop from an ovary, however; such structures are “false fruits”.
How do fruits serve as agents of dispersal?
Some may be carried away by the wind. Many attract animals that will eat the fruit and pass the seeds through their digestive systems, then deposit the seeds in another location. Cockleburs are covered with stiff, hooked spines that can hook into fur (or clothing) and hitch a ride on an animal for long distances.
What invention did cockleburs inspire?
The cockleburs that clung to the velvet trousers of a Swiss hiker, George de Mestral, inspired his invention of the loop and hook fastener he named Velcro.
Which genes are frequently chosen for DNA alignment analysis?
The genes encoding the ribosomal RNA from the small 18S subunit and plastid genes are frequently chosen for DNA alignment analysis.
What are conifers?
The dominant phylum of gymnosperms with the most variety of trees.
What is a cycad?
A gymnosperm that grows in tropical climates and resembles a palm tree; member of the phylum Cycadophyta.
What does it mean to be dioecious?
Describes a species in which the male and female reproductive organs are carried on separate specimens. Literally, “two homes” or unisexual.
What is a ginkgophyte?
A gymnosperm with one extant species, the Ginkgo biloba: a tree with fan-shaped leaves.
What is a gnetophyte?
A gymnosperm shrub with varied morphological features that produces vessel elements in its woody tissues; the phylum includes the genera Ephedra, Gnetum and Welwitschia.
What is a gymnosperm?
A seed plant with naked seeds (seeds exposed on modified leaves or in cones).
What is an integument?
A layer of sporophyte tissue that surrounds the megasporangium, and later, the embryo.
What is a megasporocyte?
A megaspore mother cell; larger spore that germinates into a female gametophyte in a heterosporous plant.
What is a microsporocyte?
A smaller spore that produces a male gametophyte in a heterosporous plant.
What does it mean to be monoecious?
Describes a species in which the male and female reproductive organs are on the same plant. Literally, “one home” or bisexual.
What is an ovulate cone?
A cone containing two ovules per scale.
What is a strobilus?
A plant structure with a tight arrangement of sporophylls around a central stalk, as seen in cones or flowers; the male strobilus produces pollen, and the female strobilus produces eggs.
What are some characteristics of gymnosperms?
Gymnosperms are paraphyletic, and their characteristics include naked seeds, separate female and male gametes, pollination by wind, and tracheids (which transport water and solutes in the vascular system).
In what ways are gymnosperm seeds “naked”?
Gymnosperm seeds are not enclosed in an ovary; rather, they are exposed on cones or modified leaves.
In which types of environments do gymnosperms predominate?
They are adapted to live where fresh water is scarce during part of the year, or in the nitrogen-poor soil of a bog. They are still the prominent phylum in the coniferous biome or taiga, where the evergreen conifers have a selective advantage in cold and dry weather. Evergreen conifers continue low levels of photosynthesis during the cold months, and are ready to take advantage of the first sunny days of spring.
What is a disadvantage that conifers face?
One disadvantage is that conifers are more susceptible than deciduous trees to infestations because conifers do not lose their leaves all at once. They cannot, therefore, shed parasites and restart with a fresh supply of leaves in spring.
What are some reproductive characteristics of gymnosperms?
The life cycle of a gymnosperm involves alternation of generations, with a dominant sporophyte in which the female gametophyte resides, and reduced gametophytes. All gymnosperms are heterosporous. The male and female reproductive organs can form in cones or strobili. Male and female sporangia are produced either on the same plant (monoecious), or on separate plants (dioecious).
What are some reproductive characteristics of pine trees?
Pine trees are conifers and carry both male and female sporophylls on the same mature sporophyte. Therefore, they are monoecious plants. Like all gymnosperms, pines are heterosporous and generate two different types of spores: male microspores and female megaspores.
How does pollination and fertilization occur in conifers?
In the male cones, or staminate cones, the microsporocytes give rise to pollen grains by meiosis. In the spring, large amounts of yellow pollen are released and carried by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube develops slowly, and the generative cell in the pollen grain divides into two haploid sperm cells by mitosis. At fertilization, one of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of the egg cell.
How does a female gametophyte develop in conifers?
Female cones, or ovulate cones, contain two ovules per scale. One megaspore mother cell, or megasporocyte, undergoes meiosis in each ovule. Three of the four cells break down; only a single surviving cell will develop into a female multicellular gametophyte, which encloses archegonia.
How do pine tree seeds develop after fertilization?
Upon fertilization, the diploid egg in the archegonium will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte that will provide nutrients, and the embryo itself.
