Chapter 26: Seed Plants

Describe the two major innovations that allowed seed plants to reproduce in the absence of water

Both seeds and pollen distinguish seed plants from seedless vascular plants. These innovative structures allowed seed plants to reduce or eliminate their dependence on water for gamete fertilization and development of the embryo, and to conquer dry land.

Explain when seed plants first appeared and when gymnosperms became the dominant plant group

• The first reliable record of gymnosperms dates their appearance to the transition of Devonian to Carboniferous period, about 319 million years ago

• However, by the early (Triassic, c. 240 MYA) and middle (Jurassic, c. 205 MYA) Mesozoic era, the landscape was dominated by the true gymnosperms.

Discuss the purpose of pollen grains and seeds

• Pollen grains – Male gametophytes containing sperm (1n) and encased in a protective coat, and can travel very far​

• Seed offers the embryo protection, nourishment, and a mechanism to maintain dormancy for extended periods – could be tens of thousands of years

Describe the significance of angiosperms bearing both flowers and fruit

The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range.

SEED PLANTS: introduction​

• The first plants to colonize land were most likely related to the ancestors of modern day mosses (bryophytes), then liverworts followed—and then primitive vascular plants (pterophytes)

  • Mosses (bryophytes) —> Liverworts —> Primitive vascular plants (pterophytes)

• Gymnosperms and angiosperms DOES NOT require water for the process of fertilization

• Dominant sporophyte generation

  • reduction in the size of the gametophyte from a conspicuous structure to a microscopic cluster of cells enclosed in the tissues of the sporophyte

  • Heterosporous- producing two types of spores: megaspores (female) and microspores (male).

    • Because gametophyte maturation depends on water and nutrient supply from the dominant sporophyte tissue, they are not free-living, as are the gametophytes of seedless vascular plants.

  • SEEDS AND POLLENS

  • Pregymnosperms (naked seeds)—> Earliest Seed plants —> Gymnosperms

Evolution of Gymnosperms

• They produced their seeds along their branches in structures known as cupules (protects the ovule)

• First gymnosperms – Devonian period

• The Ginkgoales, a group of gymnosperms with only one surviving species—the Ginkgo biloba—were the first gymnosperms to appear during the early Permian. (ENVR went from wet to dry **where seed plants thrive)

• Expansion seen in the Mesozoic era to supplanting ferns—> THEIR era of dominance

• The Jurassic period was as much the age of the cycads (palm-tree-like gymnosperms) as the age of the dinosaurs.

• While Angiosperms still dominate most biomes today, gymnosperms dominante ecosystems like taiga (boreal forests) and alpine forests at higher mountain elevations (Adapted well to cold and dry conditions

Earliest known gymnosperm

“seed fern” Elkinsia polymorpha

Seed fern leaf. This fossilized leaf is from Glossopteris, a seed fern that thrived duringthe Permian age (290–240 million years ago)

Seeds and Pollen as an evolutionary adaptation to dry land

• In seed plants, female gametophyte consists a few cells - the egg and supportive cells including the endosperm producing cells for the support of the embryo

• After fertilization, the diploid zygote produces an embryo that will grow into the sporophyte when the seed germinates

• Storage tissues to sustain growth and protective coat give seed their superior evolutionary advantage

• Several layers of hardened tissue prevents desiccation​

  • Abscisic acid helps to maintain the state of dormacy

• Dispersal through wind, floating, vectors – animals​

• Male gametophytes – pollen grains contain a few cells​

  • Distributed by water, wind, animal pollinator​

  • Protected from desiccation​

  • After reaching the female gametophyte – it grows a tube to deliver a male nucleus to the egg cell​

  • Sperms of all modern gymnosperms and angiosperms do not have flagella but the primitive ones might (Cycads, Ginkgo etc.)​

Evolution of angiosperms, flowers and fruits as an evolutionary adaptation

• Angiosperms (seeds in a vessel) produce flowers containing male and/or female reproductive structures

• Appeared in the lower Cretaceous period​

• Probably NOT derived from gymnosperm ancestors; They are a sister clade (a species and it descendants) that developed in parallel with the gymnosperms)

• Flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range

• No consensus on the origin of angiosperms (conflicting information of differing people)

• Most primitive is the Amborella trichopoda

• Modern angiosperms are classified as monocots and eudicots based on the structure of their leaves and embryos

• Angiosperms produce their gametes in separate organs, which are usually housed in a flower

• Mdiverse phylum on Earth after insects

• Flowers have all shapes, colors, smell and arrangment

• Most flowers have mutualistic pollinators (coevolution)

  • Honeybee and flower

  • Hummingbird and flower

Most primitive living angiosperm

Amborella trichopoda

Angiosperms and development of fruit

• Following fertilization of the egg, the ovule grows into a seed

• The surrounding tissues of the ovary thicken to form a FRUIT that will protect the seed

  • Not all fruits develop from the ovary. Some develop from tissues adjacent to the ovary​ (“false fruits" or pseudocarps,)

• Fruits are varied – tomatoes, green peppers, corn, avocados etc​

• Fruits are agents of dispersal – wind, animals eat them

• Cockleburs!!! Hook and spines (think of Velcro)​

Enterprising Swiss hiker, George de Mestral

Inspired his invention of the loop and hook fastener he named Velcro

Plant phylogeny.This phylogenetic tree shows the evolutionary relationships of plants.

Discuss the type of seeds produced by gymnosperms, as well as other characteristics of gymnosperms

• Characteristics of the gymnosperms include naked seeds

  • separate female and male gametophytes

  • pollen cones and ovulate cones

  • pollination by wind and insects

  • tracheids (which transport water and solutes in the vascular system)

Identify the geological era dominated by the gymnosperms and describe the conditions to which they were adapted

Gymnosperms were the dominant phylum in the Mesozoic era.

They are adapted to live where fresh water is scarce during part of the year, or in the nitrogen-poor soil of a bog. Therefore, 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.

List the four groups of modern-day gymnosperms and provide examples of each

• Conifers are the dominant phylum of gymnosperms, with the greatest variety of species

• Cycads thrive in mild climates, and are often mistaken for palms because of the shape of their large, compound leaves.

• The single surviving species of the ginkgophytes group is Ginkgo biloba

• The phylogenetic position of the gnetophytes is not currently resolved.

Describe the life cycle of a typical gymnosperm

The sporophyte (2n) phase is the longest phase in the life of a gymnosperm. The gametophytes (1n)—produced by microspores and megaspores—are reduced in size. It may take more than a year between pollination and fertilization while the pollen tube grows towards the growing female gametophyte (1n), which develops from a single megaspore. The slow growth of the pollen tube allows the female gametophyte time to produce eggs (1n).

Conifer life cycle. This image shows the life cycle of a conifer. Pollen from male cones blows up into upper branches, where it fertilizes female cones. The megaspore shown in the image develops into the female gametophyte as the pollen tube slowly grows toward it, eventually fusing with the egg and delivering a male nucleus, which combines with the female nucleus of the mature egg.

At what stage does the diploid zygote form?

1. when the female cone begins to bud from the tree

2. at fertilization

3. when the seeds drop from the tree

4. when the pollen tube begins to grow

2. at fertilization

Gymnosperms

• “Naked seeds”— diverse

• Anthophyte hypothesis suggests they are the sister group of angiosperms, while the “netifer hypothesis” suggests monophyletic group

• Separate female and male gametes, pollination is by wind (pollen cones (males) and ovulate cones (females))

• Possession of tracheids for the transportation of water and solutes in the vascular system​

• Seeds not enclosed in an ovary but are partially sheltered by modified leaves known as sporophylls​

• Strobilus – a tight arrangement of sporophylls around a central stalk​

• Some seeds are surrounded by sporophyte tissues upon maturation – the layer of sporophyte tissue that surrounds the megasporangium and the embryo is called the integument

• Dominant in the Mesozoic era, it survived without fresh water, nitrogen-poor soil​

• Prominent in the coniferous biome or taiga, conifers have a selective advantage in cold and dry weather​

• Alternation of generations with a dominant sporophyte​

• Heterosporous, male and female organs can form in cones or strobili​

• Male and female sporangia are produced either on the same plant (monoecious) or on separate plants (dioecious, “two homes” or unisexual)

• Thrives well with scarcity of water, at least for some part of the year​

• Can do well with nitrogen poor soil

Life cycle of a conifer (cone-bearing) pine trees

• Carry both male and female sporophylls on the same mature sporophyte (monoecious)​

• Heterosporous (male microspores and female megaspores). Different strobili (small male cones and larger female cones)​

  • Male or Staminate cones – microsporocytes undergo meiosis to produce haploid microspores that give rise gametophytes or “pollen grains” by mitosis​

  • During spring large amounts of pollens are released, some male gametophytes would land on female cones​

  • Pollination is the initiation of a pollen tube growth​

  • Pollen tube develops slowly and the generative cell in the pollen grain produces 2 haploid sperm or generative nuclei by mitosis​

  • One of the haploid sperm nuclei will unite with haploid nucleus on an egg​

    • Female or ovulate cones contain two ovules per scale with each ovule having a narrow passage that opens near the base of the sporophyll​

    • The passage is known as the micropyle through which the pollen tube will later grow​

    • One megaspore mother cell or megosporocyte undergoes meiosis in each ovule​

    • Thereafter 3 of the 4 cells break down, leaving only one cell which will develop into a female multicellular gametophyte that encloses the archegonia (contains a single large egg)​

    • One of the sperm nuclei from the pollen tube will enter the egg and fuse with the egg nucleus as the egg matures​

    • Upon fertilization, the diploid egg gives rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant​

    • Several eggs may be formed and fertilized but usually we have only a single surviving embryo in each ovule​

    • Fertilization and seed development may take up to 2 years in pine trees after pollination​

    • The seed formed has 3 generations of tissues – the sporophyte tissue, the gametophyte tissue (providing nutrients) and the embryo

Diversity of Gymnosperms

Classified into 4 phyla: Coniferophyta, Cycadophyta, Ginkgophyta and​ Gnetophyta (more like the angiosperms due to advanced xylem tissue)

Secondary cambium – cells for vascularization and seed development pattern​

Which phylum of gymnosperms is considered closest to angiosperms?

Gnetophyta

Conifers

• Dominant gymnosperms with the greatest variety of species​

• Tall, scale-like or needle-like leaves​

• Reduced water evaporation from leaves (thin shape and thick cuticle)​

• Snow slides off easily from the needle-shaped leaves​

• Adaptation to cold and dry weather – high altitude and cold climates​

• Pines, spruces, firs, cedars, sequoias and yews​

• A few are deciduous and shed leaves in the fall (Bald cypress, dawn redwood etc.)​

• Good for paper pulp and timber​

• Contains tracheids and no vessel elements. Referred to as soft wood​

Tracheids

Cycads

• Thrives in mild climates, mistaken often as palms due to large compound leaves​

• Large cones/strobili​

• Easily pollinated by beetles rather than wind​

• Facing extinction and are protected​

• Often used as ornamental plants in gardens in the tropics and subtropics

Conifers. Conifers are the dominant form of vegetation in cold or arid environments and at high altitudes. Shown here are the (a) evergreen spruce Picea sp., (b) juniper Juniperus sp., (c) coastal redwood or sequoia Sequoia sempervirens, and (d) the tamarack Larix laricina. Notice the deciduous yellow leaves of the tamarack.

Cycad. This cycad, Encephalartos ferox, has large cones and broad, fern-like leaves.

Ginkophytes

• Only one single surviving specie known as the Gingko biloba​

• Fan-shaped leaves, dichotomous venation pattern – turn yellow in autumn and fall from thetree​

• CULTIVATED traditionally by Chinese Buddhist monks in monasteries for generations​

• Very resistant to pollution and therefore planted in public places​

• Male and female organs are produced on separate plants​

• Gardeners prefer to plant male trees because the seeds from the female produce off-putting smell.​

Ginkgo. This plate from the 1870 book Flora Japonica, Sectio Prima (Tafelband) depicts the leaves and fruit of Gingko biloba

Gnetophytes

• Closest relatives to modern angiosperms which include the genera Ephedra, Gnetum and Welwitschia (3 living genera are quite dissimilar- may suggest that the group is not monophyletic)​

• Broad leaves​

  • Ephedra is common in the dry areas of the west coast of the United States and Mexico​

  • Gnetum species are found in some parts of Africa, South America and Southeast Asia and include trees, shrubs and vines​

  • Welwitschia is common in the Namib desert, produces two leaves which grow throughout the life of the plant (some plants are up to 100 years old)​

• Ephedra’s small scale-like leaves are the source of the compound Ephedrine used as a potent decongestant in medicine​

• Possess vessel elements suggesting a close relationship to angiosperms