1. Plant Diversity and Reproduction
Chapters 30 & 33
As plants moved onto land, the challenges of carrying out photosynthesis in air were matched with the difficulties of completing their life cycle. Land-plant ancestors had relied on water currents to carry sperm to egg and disperse their offspring.
The first plants were confronted with the challenge of moving gametes and offspring through the air. Air is less buoyant than water, provides a poor buffer against changes in temperature and UV radiation, and increases the risk of drying out.
The modification of land-plant life cycles is a major theme in plant evolution, and as plants diversified on land, coevolution with animals emerged as a second major theme.

2. Phylogeny of Land Plants
33.1 Plant diversity is dominated by angiosperms, which make up about 90% of all extant plant species.
Over 400,000 extant species
Over 90% are angiosperms
Land plants date back to 465 mya
Angiosperms are newest from140 mya
Evolution of angiosperms resulted in rapid plant diversity
Moist tropical rain forests dominated by angiosperms provided new types of habitats into which other plants could evolve
This phylogenetic tree illustrates the number of species within each major branch of land plants. Remarkably, 90% of the plants present today are flowering plants (angiosperms) and yet angiosperms are relative newcomers in plant evolution. They appeared in the fossil record about 140 million years ago.
The oldest known evidence of land plants is found in rocks approximately 465 million years old. So for more than 300 million years, terrestrial vegetation was made up of plants other than angiosperms, yet today they make up 90% of the plant population.
90% of all plants

3. Land Plant Phylogeny Showing Fertilization and Dispersal
30.1 The plant life cycle evolved in ways that enhance the ability to unite gametes and disperse offspring on land.
Plants did not evolve the ability to reproduce on land all at once. Early lineages evolved the capacity to disperse their offspring through the air, but they still needed water for fertilization.
Early land plants evolved a life cycle in which one generation or phase of the life cycle released sperm into a moist environment and the following generation dispersed offspring through the air.
Molecular sequence comparisons show that two groups of green algae are most closely related to land plants, Coleochaete and Chara. Like all sexually reproducing eukaryotes, these algae alternate between a diploid (2n) phase and a haploid (1n) phase.
Early land plants evolved a life cycle in which 1 generation (phase) of the life cycle released sperm into a moist environment and the following generation dispersed offspring through air. Called Alternation of Generations

4. Coleochaete and Chara
Coleochaete and Chara have a multicellular body that consists of haploid cells--unlike the multicellular body of animal cells, which consists of diploid cells. The diploid phase is generated by the fusion of haploid gametes to form a zygote, and the haploid phase is generated from diploid cells by meiosis.
2 groups of green algae are most closely related to land plants, Coleochaete and Chara.
Have one multicellular haploid generation

5. LIFE CYCLE OF GREEN ALGAE
They have multicellular bodies with haploid cells
The only diploid cell is the zygote
Both fertilization and dispersal take place in water
Specialized cells of the haploid body produce haploid eggs and sperm by mitosis within multicellular reproductive organs. The egg is retained within the female reproductive organ and the sperm are released into the water.
Fertilization results when one egg and one sperm fuse to form a diploid zygote.
The zygote then undergoes meiosis to produce haploid cells that give rise to a new multicellular haploid generation.
In Chara, the zygotes disperse, carried by water currents until they settle and undergo meiosis. In both Chara and Coleochaete, both fertilization and dispersal take place in water, making them very dependent on water. If these algae are left on dry land during a drought, their photosynthetic bodies shrivel. However, they are able to survive because the zygotes of these plants form a protective wall that allows the cell inside to tolerate exposure to air. When water becomes available again, they undergo meiosis, and the resulting haploid cells escape from their protective coat.

6. Earliest Forms of Land Plants
The bryophytes (hornworts, liverworts, and mosses) make up the earliest branches on the phylogenetic tree of land plants. The moss Polytrichum commune, common in forests and along the edge of fields, illustrates how the movement to land was accompanied by the evolution of two multicellular generations: one specialized for fertilization and the other for dispersal.

7. ALTERNATION OF GENERATIONS
Plant Body
Sporophyte (2n)
Gametophyte (n)
Sporangium
Spores (n)
Gametangia
Gametes
Sperm(n) + Egg(n)
Embryo(2n)
Jacket
Anthridium (male)
Archegonium (female)
Repro.

8. ALTERNATION OF GENERATIONS
All land plants have life cycles with an alternation of gametophyte (“gamete-plant”) and sporophyte (“spore-plant”) generations
1 generation is the gametophyte: the multicellular organism with haploid cells (n)
other generation is the sporophyte: the multicellular organism with diploid cells (2n)
the 2 generations alternate, each producing the other

9. Alternation of Generations Cont’d
The 2 plant forms are named for the type of reproductive cells they produce:
Gametophytes (n)
Form gametes by mitosis in gametangia
Gametes: haploid reproductive cells that cannot develop directly into organisms
Must unite sperm and egg gametes in water to form diploid zygote
In mosses only, the haploid gametophyte is the dominant generation
Sporophytes (2n)
Produce spores by meiosis in sporangia
Spore: haploid reproductive cell, but one that develops directly into an organisms (the haploid gametophyte) without fusing with another
In all other plant groups (other than mosses/bryophytes), the sporophyte generation is the dominant one

10. ALTERNATION OF GENERATIONS
Multicellular n n n 2n
Multicellular 2n

11. QUICK CHECK
Give two reasons why spores are better suited for dispersal than are sperm.

12. ANSWER 1. Spores can be dispersed by the wind.
2. Wall of spores contain sporopollenin….a tough resistant covering.

13. Bryophytes
33.2 Bryophytes diverged before the evolution of vascular plants, and they grow in environments where the ability to obtain water from the soil does not provide a disadvantage.
Bryophytes
Seedless vasc
Liverworts, mosses, and hornworts are referred to as bryophytes although they do not all descend from the same ancestor. Because they share so many features, they are often discussed as a group.
Bryophytes are the first plant lineages to diverge after plants moved onto land. They provide us with insights as to how plants gained a foothold on the terrestrial environment.
Bryophytes diverged before the evolution of lignified xylem. They add to our understanding of the evolutionary history of plants by showing how plants that lack xylem and phloem are able to survive in terrestrial environments alongside vascular plants. They have continued to evolve as the conditions on land have changed over the past 400+ million years.
Mosses are the most widely distributed of the bryophyte lineages and the most diverse, with about 15,000 species. Liverworts (8000 species) and hornworts (100 species) are less widespread and less diverse.
Seed Vasc
Paraphyletic group; 3 types of bryophytes:;Mosses, Liverworts, Hornworts
Small, simple and tough plants; Have either a flattened thallus or upright leafy

14. Bryophyte Diversity
“Bryon” means moss; small and tough plants; cannot retain water and cannot deliver water to other plant parts since they do not have vasculature
Therefore, need moist environment
Main component of life cycle ….the mat of moss….is gametophyte (haploid multicellular generation that produces gametes). Sporophytes are dependent on gametophyte.
They live in all temperatures and altitudes, only plant to live in Antarctica.
They do not need roots or soil for water; so they live on rocks and tree trunks/branches
Bryophytes are small, most only a couple of centimeters in height. The major constraint on their size is thought to arise from their mode of fertilization. Their sperm are released into the environment and must swim through films of surface water or be transported by the splash of a raindrop to meet a female gamete. Sperm cannot travel far in this manner and need the presence of water. A few mosses can grow to more than a half a meter, but these are found in the understory of very wet forests.
Bryophytes also have simple bodies. Some produce only a flattened photosynthetic structure called a thallus (the liverwort and hornwort in photos b and c). Others consist of slender stalks and have a leafy appearance (the moss in photo a), but these structures are only one to several cells thick and lack internal air spaces or a water-conducting system.
These bryophyte bodies represent the haploid generation. The diploid, spore-producing generation remains physically attached to the gametophyte, and nutritionally dependent upon it. In most bryophytes, the sporophyte extends several centimeters above the gametophyte in order to increase the chances of the spores being dispersed through the air. In mosses and liverworts, the sporophyte is short lived, drying out after the spores are dispersed. However, in hornworts, the sporophyte can live nearly as long as the gametophyte because it can produce new cells at its base.
Bryophytes do not have lignified xylem conduits, so they must absorb water and CO2 through their surfaces. Their small size might suggest that they are delicate but in reality they are able to withstand all environments. They can be found from the equator to both the latitudinal and altitudinal limits of vegetation, and from swamps to deserts. They are the only plants that grow on the continent of Antarctica.
Nevertheless, because bryophytes are so small, they are poor competitors for light and space. They thrive in local environments where roots do not provide an advantage. Many live on the branches and trunks of trees rather than the ground, making them epiphytes. They are well suited for this type of growth because they are not dependent on the soil as a source of water.

15. Bryophyte Specialization
Given that bryophytes have evolved in parallel with vascular plants, it is not surprising that they have evolved similar solutions to the same environmental challenges, a process known as convergent evolution. Examples:
The yellow moose-dung moss depends on insects to transport its spores (left photo).
In some mosses and liverworts, specialized cells are present for the transport water and carbohydrates. These evolved independently of the xylem and phloem found in vascular plants. These internal transport cells are found in the largest of the bryophytes (b).
Peat moss covers large regions known as peat-lands, and has many practical uses, including fuel

16. Sphagnum Moss Dominant plant of peat bogs
Produces water-holding cells that allow it to soak up water and to acidify environment
Helps slow decomposition, so much carbon build-up
Bryophytes are epiphytes: a plant that grows high in the canopy of other plants, or on branches or trunks of trees, without contact with the soil
In most ecosystems, bryophytes make only a small contribution to the total biomass. The one exception is peat bogs, wetland in which dead organic matter accumulates. A major component of peat bogs is sphagnum moss. These mosses play a key role in creating wet and acidic conditions that slow rates of decomposition. They have specialized cells that hold onto water, much like a sponge, and they secrete protons that acidify the surrounding water.
Peat bogs occupy 2−3% of the total land surface, but they store large amounts of organic carbon—on the order of 65 times the amount released each year from the combustion of fossil fuels.

17. BRYOPHYTES Bryophytes/Nonvascular plants: Mosses Liverworts Hornworts
Haploid gametophyte dominant
For fertilization
Polytrichum has a photosynthetic body made up of haploid cells.
The haploid body forms gametes by mitotic division of specialized cells.
Sperm travel to eggs retained within reproductive organs, and fusion of egg and sperm gives rise to the diploid zygote.
3. The zygote does not undergo meiosis and it does not disperse. Instead, the zygote is retained within the female reproductive organ, where it divides repeatedly by mitosis to produce a new multicellular generation made of diploid cells.
4. Some cells of the diploid body undergo meiosis, giving rise to spores. The spores disperse and give rise to a new haploid generation.
Because the diploid multicellular generation gives rise to spores, it is called the sporophyte. Because the haploid multicellular generation gives rise to gametes, it is called the gametophyte.
The resulting life cycle where a haploid gametophyte and a diploid sporophyte alternate is called alternation of generations, and it describes the basic life cycle of all land plants.

18. GAMETOPHYTE AND SPOROPHYTE IN BRYOPHYTES
In mosses, the sporophyte (2n) grows directly out of the gametophyte’s (n) body (dependent sporophyte)
Sporophyte for mosses is for dispersal
Late in the growing season of Polytrichum, the green tufts of this moss sprout an extension (the brown capsule at the end of a cylindrical stalk). This is the sporophyte, which originated from a fertilized egg. Because the fertilized egg was retained within the female reproductive organ, the sporophyte grows directly out from the gametophyte’s body.
Here, you can see both the gametophyte (green) and the sporophyte (brown). The gametophyte is photosynthetically self sufficient. In contrast, the sporophyte obtains water and nutrients needed for its growth from the gametophyte.

19. SPORANGIUM AND SPOROPOLLENIN
Multicellular jacket on sporophyte that produces/protects spores
Sporopollenin
Complex mixture of polymers
A tough resistant covering to spores
Provides environmental protection to spores
The multicellular sporophyte enhances the ability of plants to disperse on land. The capsule at the top of the Ptychomitrium sporophyte shown here is a sporangium, a structure in which many thousands of cells undergo meiosis, producing large numbers of haploid spores.
Spores are ideally suited for transport through the air because, being small, they can be carried for thousands of kilometers by the wind. At the same time, they are at risk of being washed from the air by raindrops. The sporangia of many bryophytes release their spores only when the air is dry.

20. quick check
In what ways are spores similar to gametes, and in what ways do spores and gametes differ?

21. ANSWER Both: Gametes: Spores: Unicellular and haploid
Short-lived and requires hydration
Must fuse with another gamete
Spores:
Can be long-lived; can survive exposure to air because of sporopollenin
Can grow into a new individual once dispersed

22. Fossil Record of Earliest Vascular Plants
33.3 Spore-dispersing vascular plants are small, often epiphytic plants that grow in moist environments.
Seedless vascular plants:
Lycophytes
Ferns and horsetails
First plants to grow tall
Have tubes; xylem for water and phloem for food
Dominant lifecycle is diploid sporophyte, with a tiny independent gametophye
Seedless vascular plants dominated early forests
Their growth helped global cooling at end of Carboniferous period
Decaying remnants of ferns/first forests eventually became coal
Vascular plants can be divided into two groups according to how they complete their life cycle:
Lycophytes, ferns, and horsetails disperse by spores and rely on swimming sperm for fertilization.
Gymnosperms and angiosperms are seed plants that disperse by the movement of their seeds and pollen.
Fossils recovered from a single site in Scotland document key stages in the evolution of vascular plants in chert, a mineral made of silica. Early vascular plants were small photosynthetic structures that branched repeatedly, forming sporangia at the tips of short side branches. These plants had no leaves, and their only rooting structures were small hairlike extensions on the lower parts of stems that ran along the ground. The stems had a cuticle with stomata, and a tiny cylinder of vascular tissue ran through the center. Some fossils of branched sporophytes show no evidence of vascular tissue. Evidently, plants evolved the upright stature we associate with living vascular plants before they evolved xylem and phloem.

23. SEEDLESS VASCULAR PLANTS
The lycophytes and the ferns and horsetails are the first two lineages of vascular plants. Both depend on swimming sperm for fertilization and disperse by spores that are released into the air. In this way, they are similar to the bryophytes. However, the evolution of vascular systems had a major impact on their life cycle because it allowed them to grow tall. Xylem and phloem are present in the sporophyte generation.
Gametophytes must remain small and close to the ground to increase the chances of fertilization. On the other hand, spore dispersal is enhanced by height, and spore production increases with overall size.
An important difference between the life cycles of the bryophytes and the spore-dispersing vascular plants is that, in bryophytes, the gametophyte is the photosynthetic generation, while in vascular plants, it is the sporophyte generation that dominates both in physical size and photosynthetic output.

24. SEEDLESS VASCULAR PLANTS
Ferns, horsetails, lycophytes
Depend on swimming sperm for fertilization and dispersal of spores into air
1st plants to grow tall….xylem and phloem present in sporophyte generation
Large PS diploid sporophyte is dominant generation; height is crucial for spore dispersal
Gametophytes are small and close to ground to increase chances of fertilization
Other difference with mosses:
In mosses…the gametophyte is PS generation
In vascular plants….the sporophyte dominates in physical size and PS output
The lycophytes and the ferns and horsetails are the first two lineages of vascular plants. Both depend on swimming sperm for fertilization and disperse by spores that are released into the air. In this way, they are similar to the bryophytes. However, the evolution of vascular systems had a major impact on their life cycle because it allowed them to grow tall. Xylem and phloem are present in the sporophyte generation.
Gametophytes must remain small and close to the ground to increase the chances of fertilization. On the other hand, spore dispersal is enhanced by height, and spore production increases with overall size.
An important difference between the life cycles of the bryophytes and the spore-dispersing vascular plants is that, in bryophytes, the gametophyte is the photosynthetic generation, while in vascular plants, it is the sporophyte generation that dominates both in physical size and photosynthetic output. 24

25. Early Lycophyte Early lycophytes: 300 mya
Large trees that dominated swamp forests
Todays lycophytes:
Small plants that grow in forest as epiphytes
Occur in shallow ponds
One of the fossil populations in these cherts is notably larger and more complex than the others. These are among the earliest fossils of lycophytes, an early branching group of vascular plants that can still be found from tropical rain forests to Arctic tundra. These fossils show leaf-like structures arranged on the stem in a spiral pattern. Through the stem runs a thick-lobed cylinder of xylem.

26. Giant Lycophytes
Fossils show that ancient lycophytes evolved additional features convergently with seed plants. These features included
a vascular cambium and cork cambium that enabled them to form trees up to 40 m tall and,
in some cases, reproductive structures similar to seeds.
Swamps that formed widely about 320 million years ago were dominated by tree-sized lycophytes. The vascular cambium of these lycophytes produced relatively little secondary xylem and no secondary phloem. Instead, the giant lycophytes relied on thick bark for mechanical support. Tree-sized lycophytes were markedly different from the trees familiar to us today.
These lycophytes persisted for millions of years in swampy environments alongside early seed plants; they were not outcompeted. Their demise happened when changing climates dried out swamps. When the trees died and fell over into the swamp, their bodies decomposed slowly. Over time, this dead organic matter was converted to the carbon-rich material we call coal. Thus, a major source of energy used today by humans is derived originally from photosynthesis carried out by giant lycophyte trees.

27. Woody Plant Evolution Did woody plants evolve more than once?
The vascular cambium is recorded in fossils by xylem cells in rows oriented radially in the stem or root. Thus, the giant tree lycophytes of Carboniferous coal swamps had a vascular cambium, as did extinct tree-sized relatives of the horsetails and other extinct horsetail relatives. Phylogenetic trees generated from morphological features preserved in fossils show that woody lycophytes, woody horsetail relatives, and the group of seed plants progymnosperms did not share a common ancestor that had vascular cambium.
Anatomical research shows that the vascular cambium of extinct woody lycophytes and the giant horsetails Archaecalmites and Calamites generated secondary xylem but not secondary phloem, unlike the vascular cambium of seed plants. Living horsetails do not have a vascular cambium, but fossils show that they are descended from ancestors that did make secondary xylem and have lost this trait through evolution.
Fossils and phylogeny support the hypothesis that the vascular cambium and, hence, wood evolved more than once, reflecting a strong and persistent selection for tall sporophytes among vascular plants.

28. Ferns and Horsetail Diversity
Ferns and horsetails are morphologically diverse
Ferns produce large leaves that uncoil as they grow
Horsetails have tiny leaves
Whisk ferns have no leaves at all
Ferns and horsetails form a monophyletic group that are the sister group to the seed plants. The majority of species in this group are ferns, which can be recognized by their distinctive leaves. Fern leaves can be large, although typically the photosynthetic surfaces are divided into smaller units called pinnae (f). Fern stems frequently grow underground, and only the leaves emerge into the air.
Fern leaves may have other forms, like those of aquatic ferns (e) and marattioid ferns (d).
This group also includes horsetails and whisk ferns (and the adder’s tongue fern (a)), which traditionally were considered as distinct groups because of their unique body organizations. Whisk ferns have photosynthetic stems without leaves (b) and do not form roots. They look similar to fossils of early vascular plants. Molecular-sequence comparisons, however, support the hypothesis that whisk ferns are the simplified descendants of plants that produced both leaves and roots and that they are members of the same lineage as ferns.
The horsetails, represented by 15 living species, produce tiny leaves arranged in whorls, giving them a jointed appearance (c). Horsetail stems are hollow, and their cells accumulate high levels of silica.
Horsetails, whisk ferns, and a few other fern groups make large sporangia similar to those produced by early vascular plants. Most ferns have a distinctive sporangium, called a leptosporangium, whose wall is only a single cell thick. Polypod ferns have leptosporangia with a line of thick-walled cells that run along the sporangium surface. When the pores mature, the sporangium dries out and these cells contract, forcibly ejecting spores into the air.
Although the fossil record shows that ferns originated more than 360 million years ago, the radiation of the polypod ferns occurred after the rise of the angiosperms. Many of the fern species present today are likely to have evolved to occupy habitats newly created by the development of angiosperm forests.

29. Life Cycle of the Fern
Alternation between smaller, free-living gametophyte and a taller, vascularized sporophyte
This life cycle illustrates the alternation between a free-living gametophyte generation that is small and a taller, vascularized sporophyte generation.
This fern sporophyte appears to consist entirely of leaves because the stem grows underground. The leaf contains tiny brown packets along its lower margin. These are sporangia, and each contains diploid cells that undergo meiosis to generate haploid spores. Like mosses, the spores become covered by a thick wall containing sporopollenin. The sporangium wall has a distinct ridge of asymmetrically thickened cells that when dry produce a motion like that of a slingshot that hurls the spores away from the leaf surface to be carried off by air currents.
These spores germinate to produce the haploid gametophyte generation. Typical of most ferns, the gametophyte is less than 2 cm long and only one to a few cells thick.
The union of a male and a female gamete forms a diploid zygote, which is supported by the gametophyte as it begins to grow. Eventually, it forms leaves and roots that allow it to become a physiologically independent, diploid sporophyte.
Ferns, as well as the other spore-dispersing plants (lycophytes, horesetails, bryophytes), release swimming sperm and are able to reproduce only when conditions are wet.

30. Quick Check
In what ways is fern reproduction similar to moss reproduction? In what ways is fern reproduction different?

31. ANSWER Both: Mosses: Ferns: Exhibit an alternation of generation
Release of swimming sperm
Dispersal of spores
Mosses:
The sporophyte is completely dependent on the gametophyte (dominant generation)
Ferns:
The sporophyte (dominant generation) is only initially dependent on the gametophyte and eventually becomes free-living

32. Advantages of Gamete and Offspring Dispersal
Outcrossing/genetic diversity
Nutrient supply
Pathogen/parasite avoidance
The advantages of being able to disperse gametes and offspring include:
Outcrossing: Sexual fusing with a genetically different member of the population creates diverse genotypes.
Nutrient supply: Dispersal of offspring spreads apart the population so that there is not competition for nutrients within a very small area.
Pathogen/parasite avoidance: Viruses and bacteria spread easily among tightly packed populations; when dispersal of offspring spreads out a population, the encounters with pathogens are fewer.

33. LIFE CYCLE EVOLUTION IN LAND PLANTS
A major trend in evolution of land plants is a decrease in size and independence of gametophyte and a corresponding increase in dominance of sporophyte
Gametophyte
Sporophyte
Dependent on gametophyte
PS persistent
Dependent on sporophyte
PS persistent

34. 30.2 SEED PLANTS 2 monophyletic groups of seed plants:
Gymnosperms: “naked seeds”
Angiosperms: “enclosed seeds in fruit”
Both groups are sporophyte dominant with a microscopic, dependent gametophyte
Advantages of seeds:
Seeds are better spores
Survive better than unprotected spores
Can be transported long distances; winged seeds
Pollen grains are better sperm
Make water unnecessary for fertilization
Not flagellated; have wings

35. Gymnosperms
33.4 Gymnosperms produce seeds and woody stems, and are most common in seasonably cool or dry regions.
Only 2 extant seed plants:
Gymnosperms (with less than 1,000 species)
4 groups of woody plants
Cycads
Gingkos
Conifers
Gnetophytes
Angiosperms (with more than 380,000 species)
Produce seeds (naked or enclosed) and woody stems
Common in cool or dry regions
Living gymnosperms include four groups: cycads, ginkgos, conifers, and gnetophytes. Fossil records show that more than a dozen additional groups, now extinct, were once present.

36. Cycads Produce large leaves on stout, unbranched stems
They now occur in small, fragmented populations….primarily in tropics and subtropics
Insect pollinated
All form symbiotic relationships with nitrogen-fixing bacteria
Cycads and palm trees are often confused because both have unbranched stems and large leaves. However, the presence of cones instead of flowers clearly mark cycads as gymnosperms.
Cycads generally have slow photosynthetic rates and grow slowly. Their vascular cambium produces little additional xylem, and most of their bulk is made up of a large pith and cortex. They form symbiotic associations with nitrogen-fixing cyanobacteria that allow them to grow in nutrient-poor environments. The apical meristem of cycads is protected by a dense layer of bud scales, enabling them to survive wildfires.
The approximately 300 species of cycads occur most commonly in tropical and subtropical regions in fragmented distributions. Cycads rely on insects for pollination. Many are pollinated by beetles, which are attracted by chemical signals produced by the cones. The insects use the large cones for shelter, and the pollen cones provide them with food. Following fertilization, brightly colored, fleshy seeds attract a variety of birds and mammals that serve as dispersal agents.
Because of their small numbers and fragmented distribution, two thirds of all cycads are on the International Union for the Conservation of Nature’s “red list” of threatened species

37. Ginkgos
Single living species of a group distributed globally before evolution of angiosperms
Wind pollinated
Produces tall, branched trees
A second group of gymnosperms has only a single living representative: Ginkgo biloba. Like cycads, ginkgos date back about 270 million years. During the time of the dinosaurs, they were common trees in temperate forests. Like cycads, ginkgos declined in abundance, diversity, and geographic distribution over the past 100 million years.
Ginko biloba forms tall, branched trees. Its fan-shaped leaves turn a brilliant yellow in autumn. Like cycads, Ginkgo develops fleshy seeds, perhaps a feature inherited from their common seed plant ancestor.

38. Gnetophytes Small group Contains only 3 genera and few species
Gnetophytes are a small group of gymnosperms made up of three morphologically different genera:
Gnetum: found in tropical rain forests, they form woody vines or small trees with large broad leaves (left).
Welwitschia mirabilis: found only in the deserts of southwestern Africa, they produce only two strap-like leaves that elongate continuously from their base (upper right).
Ephedra: native to arid regions, these are shrubby plants with photosynthetic stems and small leaves (lower right).
Gnetophytes exhibit traits typically associated with angiosperms, notably the formation of multicellular xylem vessels and double fertilization. For many years, they were thought to be a sister group to angiosperms, but DNA analysis indicates that they are not closely related. Instead, gnetophytes are closely related to conifers. Both characteristics—vessels and double fertilization—evolved independently in this group, an example of convergent evolution.
Small group
Contains only 3 genera and few species
Independently evolved xylem vessels

39. Conifers Tallest and oldest trees on Earth Wind-pollinated
Mainly evergreen
Found primarily in cool to cold environments
The conifers are the tallest and oldest trees on Earth. They include pines, junipers, and redwoods. Most conifers are evergreen, meaning they keep their needle-like leaves year round.
Conifer xylem consists almost entirely of tracheids. As a result, its wood is strong for its weight and has relatively uniform mechanical properties. Today, conifers are used for telephone poles; they supply much of the world’s timber; and they are the raw material for paper production.
Many conifers produce resin canals in their wood, bark, and leaves that deter insects and fungi. Conifer resins are harvested and distilled to produce turpentine and other solvents. Conifer resins are the source of a number of chemicals, including the drug taxol used in the treatment of cancer.
Pollen is produced in small cones, and ovules develop in larger cones that mature slowly as the fertilized ovules develop into seeds. Conifers are wind pollinated, and most species rely on wind for seed dispersal. Some conifers produce fleshy and often brightly colored tissues associated with their seeds that attract birds and other animals. For example, in junipers, the entire seed cone becomes fleshy. Juniper seed cones are used as seasoning, giving gin its distinctive flavor.
Conifers dominate the vast boreal forests of Canada, Alaska, Siberia, and northern Europe. They generally increase in abundance as elevation increases and are common in dry areas such as the western parts of North America and much of Australia. One hypothesis to explain the success of gymnosperms in these environments is that angiosperms lose their competitive advantage in water transport because their xylem vessels must be smaller to avoid cavitation.

40. GYMNOSPERMS (Tree) SPOROPHYTE Male cones Male Gametophyte (pollen)
On pollen cones
Wind Pollinated
Naked seed
Female cones
Female Gametophye (ovule)
On ovulate cones

41. GYMNOSPERMS
Gymnosperms bear “naked seeds” (not in fruit), typically on cones
Key features of life cycle:
Dominance of sporophyte generation
Development of seeds from fertilized ovules (female gametophyte)
Role of pollen (male gametophyte) in transferring sperm to ovules
I.e. conifers (pine trees), ginkos, cycads
Ovule: haploid female gametophyte; found on ovulate cones on the tree (sporophyte)
The ovule will become the seed when pollinated
Pollen grains: haploid male gametophytes; found on pollen cones
Airborne, resistant sperm cells that lack flagella; have wings instead
Fertilization by pollen transforms ovule into seed
Gymnosperms are wind pollinated

42. Seed Production vs. Spore Production
In seed-producing plants:
male gametes are never exposed to the environment
the relationship between sporophyte and gametophyte is reversed from that in bryophytes: the gametophyte is reduced to a few cells dependent on the sporophyte.
seeds are produced, which are able to disperse away from the parent plant.

43. The evolution of pollen liberated seed plants from the need to release swimming sperm into the environment. Sperm-producing gametophytes are transported through the air in pollen, and fertilization can take place in the absence of external pools of water.
In pines, and all vascular plants, the conspicuous multicellular generation is the diploid sporophyte.
As they mature, pines form reproductive structures called cones. Pines develop two types of cones:
ovule cones, which produce female gametophytes and female gametes
pollen cones, which produce male gametophytes and male gametes.
2. In pines, each spore produced within a pollen cone divides mitotically to form a multicellular male gametophyte (pollen) inside the spore wall. In pine, the male gametophyte consists of only four cells at the time the pollen is shed from the parent plnt.
3. Pine pollen is released into the air and transported by wind. For fertilization to occur, a pollen grain must land on an ovule cone, where the female gametes or eggs are produced.
4. Ovule cones contain sporangia that each produce four spores; three spores spontaneously abort, leaving a single functional spore inside. This spore undergoes repeated mitoses to form a multicellular gametophyte consisting of a few thousand haploid cells, one or more of which differentiate as eggs. This female gametophyte fills the sporangium, surrounded and protected by tissues that envelop the sporangium wall. This structure is the ovule.
5. Fertilization takes place when the pollen is carried by the wind to an ovule. For sperm to reach the egg, the male gametophyte must produce a pollen tube that grows outward through an opening in the sporopollenin coat to the female gametophyte.
Life Cycle of Pine 43

44. Seed Structure Food supply Developed from 2n sporophyte New sporophyte
A seed contains three layers from three generations:
On the outside is the protective seed coat, which is formed from tissues that surround the sporangial wall and is a product of the diploid sporophyte.
The center is the embryo, which developed from the zygote and represents the next sporophyte generation.
The embryo is surrounded by the haploid female gametophyte. which provides the raw materials that support the growth of the embryo.
The combination of low metabolic activity and stored resources allows seeds to survive for long periods of time, years in some cases. Seeds can also exhibit dormancy, allowing them to delay germination until the environmental conditions are favorable.
New sporophyte

45. Seed Diversity
Seeds span over 11 orders of magnitude. Here, two extremes are illustrated.
The seeds of orchids are so small (100 nanograms) that they cannot germinate successfully without obtaining nutrients from a symbiotic fungus. Small seeds tend to persist in soil until the right combination of conditions trigger germination.
The 30-kg seed of the coco-de-mer is found on islands in the Indian Ocean. Seeds of this size disperse less far, have short life spans, and often have little to no dormancy.

46. Angiosperms
33.5 Angiosperms are diversified by flowers, fruits, double fertilization, and xylem vessels. Diversity is partly explained by animal pollination.
Angiosperms reproduce quickly and with less use of resources
Why? Insect pollination and double fertilization
Allows them to reproduce in various habitats
The phylogenetic tree of angiosperms shows how diverse this group is, but it also shows that the early branching groups were not diverse. The last common ancestor of angiosperms and living gymnosperms is thought to have lived more than 300 million years ago.
To gain insight into the type of habitat in which the first angiosperms appeared, we look at where living plants from the earliest-branching lineages are found today. Most occur in the understory of tropical rain forests. This observation suggests that angiosperms may have evolved in wet and shady habitats. How could this type of environment have contributed to the evolution of flowers and xylem vessels?

47. ANGIOSPERMS
30.2 Angiosperms attract and reward animal pollinators, and they provide resources for seeds only after fertilization.
The efficiency of pollination is increased in plants pollinated by animals compared to plants that depend entirely on the wind to disperse their pollen.
Flowers are spectacularly diverse in size, color, scent, and form, yet they all have the same basic organization. Shown here are (upper left) a lady’s slipper orchid, (upper right) a magnolia, (lower left) French lavender, and (lower right) a flower from a tropical tree.

48. Angiosperm History Angiosperm diversity: Low rates of extinction
High rates of species formation
Major split between 2 diverse groups
Monocots
Eudicots
Plants having flowers can reproduce even if far apart
Allows rare species to persist and reproduce
Xylem vessels make it possible for angiosperms to have a diversity of form, and to grow toward light
The second important feature of angiosperms is the formation of wood containing xylem vessels. Angiosperm xylem contains two cell types, separating the functions of mechanical support and water transport:
Vessel elements stack to produce multicellular vessels through which water is transported.
Thick-walled, elongate fibers provide mechanical support.
This cellular specialization allows angiosperms to produce wood with larger diameter and much higher water transport capacity than what is found in tracheid-only plants. Faster water transport allows greater stomatal opening and in return higher rates of photosynthesis.
The formation of separate cell types may have given the early angiosperms greater flexibility in form, allowing them to grow toward small patches of sunlight that filtered through the forest canopy.
The fossil record provides evidence that, for the first 30−40 million years of angiosperm evolution, angiosperms were neither diverse nor ecologically dominant. In fact, fossil pollen indicates that between about 140 and 100 million years ago, gnetophytes diversified just as much as angiosperms. It was not until about 100 million years ago that angiosperm diversity began to increase at a much higher rate. Two group diverged that would come to dominate both angiosperm diversity and ecology: the monocots and eudicots.
2 diverse
groups

49. Monocots
Basswood Strips, 1/2 Inch Thick x 3/4 Inch Wide x 24 Inches Long (Pkg. of 2)
Single cotyledon: embryonic seed leaf
Vascular bundles scattered throughout stem
Parallel venation
Flower parts occur in 3’s (3, 6, 9, 12…)
Root is called a fibrous root
Do not form a vascular cambrium
i,.e. grasses, wheat, corn, rice, coconut palms, bananas, ginger and orchids
Most of our food supply comes from monocots
Monocots make up nearly one quarter of all angiosperms. They come in all shapes and sizes and are found in virtually every terrestrial habitat on Earth. The photos show (a) a desert succulent, (b) a spring wildflower, (c) bamboo, a forest grass, and (d) a species of the rain forest understory.
Monocots take their name from the fact that they have one embryonic seed leaf, whereas all other angiosperms have two. Monocots also never form vascular cambium. In monocot leaves, the major veins are typically in parallel and the base of the leaf surrounds the stem, forming a continuous sheath. This type of leaf base means that only one leaf can be attached at any node, consistent with the formation of a single seed leaf. Despite not producing a vascular cambium, monocot stems can still be quite large.
The lack of a vascular cambium has a profound impact on the way monocots form roots. Because they cannot increase their vascular capacity, monocots continuously initiate new roots from their stems. The root systems therefore are more similar to those found in ferns and lycophytes than in other seed plants. Finally, monocot flowers typically produce organs in multiples of three (3, 6, or 9 stamens), whereas eudicot flowers are more commonly in multiples of 4 or 5.
Examples of monocots include grasses, corn, rice, wheat, sugar cane, banana, yams, ginger, asparagus, pineapple, agave, and vanilla.

50. Monocot Body What may have influenced the monocot body?
Creeping, horizontal stems
Loose substrates
Flowing water
Fluctuating water levels
Monocots have a relatively poor fossil record in part because they do not form wood. Several factors may contribute to their unique body plan. One hypothesis is that monocots evolved from ancestors that produced creeping, horizontal stems as they grew along the shores of lakes and other wetlands. Today, many monocots grow in such habitats, and the features of the monocot body are well suited for environments with loose substrates, flowing water, and fluctuating water levels.
Their leaf base provides firm attachment that prevents leaves from being pulled off by flowing water. Many monocots produce strap-shaped leaves that elongate from a persistent zone of cell division and expansion located at the base of the leaf blade. By continually elongating from the base, monocot leaves can extend above fluctuating water levels. Finally, it is easier to imagine evolutionary changes that affected the vascular system occurring within an environment that makes only modest demands for water transport.
We may never know for sure the environmental context surrounding the evolution of monocots.

51. Eudicots
Pollen grains with 3 openings through which the pollen tube can grow
Diverse; majority of flowering plants
i.e. legumes, roses, cabbage, pumpkin, coffee, tea, cacao, maples, oaks, magnolias
Root called a taproot
Netlike venation
Dicot flowers have 4 or 5 petals
Vascular bundles arranged in a ring
Eudicots first appear in the fossil record about 125 million years ago. By 80−90 million years ago, most of the major groups present today can be distinguished. Today, there are estimated to be approximately 160,000 species of eudicots, which is nearly 75% of all angiosperm species.
Eudicots are well represented in the fossil record because their pollen is easily distinguished. Each eudicot pollen grain has three openings from which the pollen tube can grow, whereas pollen in all other seed plants has only a single opening.
Eudicots take their name from the fact that they produce two embryonic seed leaves. Many eudicots produce highly conductive xylem, which explains their high rates of water transport and high rates of photosynthesis. Important eudicot trees include oaks, willows, and eucalyptus.
At the other end of the size spectrum are the herbaceous eudicots. Herbaceous eudicots do not form woody stems. Instead, the aboveground shoot dies back each year rather than withstand a period of drought or cold. At the extreme are annuals, herbaceous plants that complete their life cycle in less than a year. Annuals are unique to angiosperms and almost all are eudicots. Examples of herbaceous eudicots include violets, buttercups, and sunflowers.
Most parasitic plants and virtually all carnivorous plants are eudicots, as are water-storing cacti. Other examples of eudicots include blueberries, woody shrubs, grapes, honeysuckle, apples, carrots, pumpkins, potatoes, coffee, cacao, olives, walnuts, soybeans, and tea.
The photos show (upper left) red oak leaves and acorns, (lower left) tropical passionflower vines, (center) blanksia shrubs, native to Australia, and (right) beavertail cactus.

52. ANGIOSPERMS SPOROPHYTE (FLOWER) Male gametophyte….pollen
Female Gametophye…ovule/ovary
Male gametophyte….pollen
Pollinator
Fruit with enclosed seed

53. Flower Organization 4 whorls of organs: Ovule-bearing carpals (female)
Sticky Stigma
Style
Ovary with many ovules
Ovules develop into seeds
Ovary develops into fruit
Pollen-producing stamen (male)
Anthers
contains several sporangia
in which pollen are produced
Filament
Petals
Sepals
Flowers are composed of concentric whorls of floral organs. The center two whorls are made up of ovule-producing carpels and pollen-producing stamens.
Each flower produces several carpels, but because they are often fused, it may seem as though there is only one. Each carpel has a hollow ovary at the base in which one to many ovules develop. The ovary protects the ovules from being eaten or damaged by animals. Because the ovules are enclosed, these plants are called angiosperms (meaning “vessel” and “seed”). In gymnosperms, the ovules are not enclosed in a vessel but are exposed.
The ovary makes it impossible for pollen to land directly on the ovule surface. Instead, carpels commonly have a cylindrical stalk at the top called the style. The surface on top of the style is called the stigma.

54. Germination and Pollen Tubes
Pollen is captured on the sticky stigma of carpal
Pollen is captured on the sticky or feathery stigma, where it germinates and extends its pollen tube down through the style to reach the egg. Here, the pollen tubes are visualized using a fluorescent dye.

55. Rewards to Pollinators
Flowers communicate their presence through scent and color, but in order for pollinators to be interested, there must be a reward for them. In many cases, the reward is in the form of food, like pollen in the upper left photo (butterfly) and nectar in upper-right and lower-left photos (bat, hummingbird). A reward can also take the form of chemical secretions or heated chambers.
In some cases, there is no reward at all. Instead, the flower tricks the animal pollinator. In the lower left photo, the flower has the smell and appearance of rotting flesh to attract flies.
Flowers attract animals because they provide a reward
Food, shelter, chemicals
Animals transfer pollen

56. POLLINATION AND DOUBLE FERTILIZATION IN ANGIOSPERMS
Pollination: Pollen carried by pollinator to stigma
Pollinators….bees, hummingbirds, butterflies
Fly to flower to eat nectar; get pollen on their legs from anthers
Pollen Tube Germination: Pollen lands on sticky stigma
Pollen has 2 types of cells:
Tube cell: burrows down to ovules
Generative cell: divides to form 2 sperm cells which travel down to ovule
Fertilization: 2 sperm cells enter ovule
1 sperm cell fertilizes egg cell…becomes diploid zygote
2nd sperm cell joins two polar nuclei to become triploid endosperm; nutrient for zygote
Called Double Fertilization

57. Angiosperm Life Cycle
A major trend in the evolution of plants is the decreasing size and independence of the gametophyte generation and the increasing prominence of the sporophyte generation. In angiosperms, this trend is taken even further.
The male gametophyte has only three cells: one controls the growth of the pollen tube, while the other two are male gametes, or sperm.
In most species, the female gametophyte contains only eight nuclei, arranged into six haploid cells plus a central cell containing two nuclei. One of the haploid cells gives rise to the egg, and the two nuclei within the central cell fuse either before or during fertilization to form a diploid cell.
The female gametophyte of angiosperms is too small to support the growth of the embryo, so the diploid cell of the female gametophyte comes into play.

58. Double Fertilization Unique to angiosperms
During pollination, the two sperm travel down the pollen tube and enter the ovule.
One of the sperm fuses with the egg to form a zygote.
The other sperm unites with the diploid gametophyte cell to form a triploid (3n) cell. This triploid cell undergoes many mitotic divisions, forming a new tissue called endosperm. In angiosperm seeds, it is the endosperm that supplies nutrition to the embryo.
The process by which two sperm from a single pollen tube fuse with the egg and the diploid cell is called double fertilization.
By double fertilizing, the female gametophyte provides food for the embryo. In angiosperms, endosperm development occurs only after fertilization.
Unique to angiosperms
2 sperm unite with 2 cells of female gametophyte
Formation of a 2n zygote and 3n endosperm (nourishes the zygote/embryo)

59. Endosperm
The embryo itself is small. As the seed germinates, the embryo draws resources from the endosperm to help it become established.

60. Seed Structure Food supply Developed from 2n sporophyte Endosperm (3n)
A seed contains three layers from three generations:
On the outside is the protective seed coat, which is formed from tissues that surround the sporangial wall and is a product of the diploid sporophyte.
The center is the embryo, which developed from the zygote and represents the next sporophyte generation.
The embryo is surrounded by the haploid female gametophyte. which provides the raw materials that support the growth of the embryo.
The combination of low metabolic activity and stored resources allows seeds to survive for long periods of time, years in some cases. Seeds can also exhibit dormancy, allowing them to delay germination until the environmental conditions are favorable.
New sporophyte

61. Flower to Fruit
As the fertilized egg develops to form an embryo and the endosperm proliferates around it, the ovary wall develops into a fruit. Fruits serve two functions:
They protect immature seeds from being preyed upon by animals.
They enhance dispersal once the seeds are mature.
It is essential for fruits not to be consumed before the seeds are able to withstand their disperser’s digestive tract. Immature fleshy fruits are physically tough, and their tissues highly astringent for this reason. As they ripen, the fruits are rapidly transformed in texture, palatability, and color. Ripening converts starches to sugars and loosens the connections between cell walls so that the fruits become softer.
Fertilization of an ovule triggers the development of the ovary wall into a fruit
Fruits protect immature seeds and enhance seed dispersal

62. Fruit and Seed Dispersal
Animals are the most important agents of seed dispersal.
Animals are probably the most important agents of seed dispersal. In many cases, they are attracted by the nutritious flesh of the fruit. In fleshy fruits, the seeds are protected by a hard seed coat and pass unharmed through the animals’ digestive tract. In some cases, seeds are dispersed when animals gather more seeds than they can eat. Humans also disperse seeds long distances, sometimes far outside their usual range.
(upper left) Wind-dispersed seeds of milkweed; (lower left) a squirrel with a fruit it will bury for later use, (lower right) a bird eating berries of the mountain ash, and (upper right) cocklebur fruits attached to the fur of a mule deer.