BIO 202 - CHAPTER 29
Most seedless vascular plant species are homosporous
They have one type of sporangium that produces one type of spore, which typically develops into a bisexual gametophyte, as in most ferns.
seedless vascular plants and bryophytes have what in common
flagellated sperm and are usually restricted to moist environments
main traits that characterize living vascular plants:
life cycles with dominant sporophytes, transport in vascular tissues called xylem and phloem, and well-developed roots and leaves, including spore-bearing leaves called sporophylls.
benefits of charophytes moving to land
unfiltered sunlight more plentiful CO2 nutrient rich soil initially very few herbivores and preadators
Lignified vascular tissue permitted
vascular plants to grow tall. Their stems became strong enough to provide support against gravity, and they could transport water and mineral nutrients high above the ground. Tall plants could also out- compete short plants for access to the sunlight needed for photosynthesis. In addition, the spores of tall plants could disperse farther than those of short plants, enabling tall species to colonize new environments rapidly. Overall, the ability to grow tall was a major evolutionary innovation that gave vascular plants a competitive edge over nonvascular plants, which rarely grow above 20 cm in height. Competi- tion among vascular plants also increased, and taller growth forms were favored by natural selection—such as the trees that formed the first forests about 385 million years ago.
heterosporous
wo types of sporangia and produces two kinds of spores: Megasporangia on megasporophylls produce megaspores, which develop into female gametophytes; microsporangia on microsporophylls produce the comparatively smaller microspores, which develop into male gametophytes.
Root tissues of living plants closely resemble
stem tissues of early vascular plants preserved in fossils. This suggests that roots may have evolved from the lowest belowground por- tions of stems in ancient vascular plants. ● Roots may have evolved from subterranean stems
Hornwort and moss sporophytes have
stomata- specialized pores. These pores support photosynthesis by allowing the exchange of CO2 and O2 between the outside air and the sporophyte in- terior (see Figure 10.3). Stomata are also the main avenues by which water evaporates from the sporophyte. In hot, dry con- ditions, the stomata close, minimizing water loss. liverwort sporophytes do not
how do we benefit from plants
supply oxygen and are the ultimate source of most food eaten by land animals
Bryophytes were prominent types of vegetation during
the first 100 million years of plant evolution
As we noted earlier, biologists recognize two clades of living seedless vascular plants:
the lycophytes (phylum Lycophyta) and the pterophytes (phylum Pterophyta). The lycophytes include the club mosses, the spike mosses, and the quill- worts. The pterophytes include the ferns, the horsetails, and the whisk ferns and their relatives. Because they differ greatly in appearance, the ferns, horsetails, and whisk ferns have long been considered separate phyla: phylum Pterophyta (ferns), phylum Sphenophyta (horsetails), and phylum Psilo- phyta (whisk ferns and a related genus). However, recent mo- lecular comparisons provide convincing evidence that all three groups make up a clade.
Bryophyte sporophytes compared to exant groups
they are the smallest and simplest
● The earliest fossils of vascular plants
425 million years ago
life cycle of a moss lecture
A spore germinates into a gametophyte composed of a protonema and one or more gametophores The height of gametophytes is constrained by lack of vascular tissues Rhizoids anchor gametophytes to substrate Mature gametophytes produce flagellated sperm in antheridia and an egg in each archegonium Sperm swim through a film of water to reach and fertilize the egg
lycophyte leaves
All of the lycophytes (the oldest lineage of present-day vascular plants)—and only the lycophytes— have microphylls, small, usually spine-shaped leaves sup- ported by a single strand of vascular tissue. Almost all other vascular plants have megaphylls,
Bryophyte Sporophytes cannot
Although bryophyte sporophytes are usually green and photo- synthetic when young, they cannot live independently. They remain attached to their parental gametophytes, from which they absorb sugars, amino acids, minerals, and water.
evolution of vascular plants
As plant bodies became in- creasingly complex, competition for space and sunlight prob- ably increased. As we'll see, that competition may have stimulated still more evolution in vascular plants.
many bryophyte species are found in moist habitats.
Bryophyte sperm typically require a film of water to reach the eggs. Given this requirement, it is not surprising that many bryophyte species are found in moist habitats. The fact that sperm swim through water to reach the egg also means that in species with separate male and female gametophytes (most mosses), sexual reproduction is likely to be more suc- cessful when individuals are located close to one another.
● Nonvascular plants are commonly called
Bryophytes do not form a monophyletic group (a clade) bryophytes share some de- rived traits with vascular plants, such as multicellular embryos and apical meristems, while lacking many innovations of vascular plants, such as roots and true leaves.
members of a grade vs members of a clan
But members of a grade, unlike members of a clade, do not nec- essarily share the same ancestry. even though pterophytes and lycophytes are all seedless plants, pterophytes share a more recent common ancestor with seed plants.
Phylum Lycophyta
Club Mosses, Spike Mosses, and Quillworts The giant lycophyte trees became extinct when Earth's climate became drier at the end of the Carboniferous period. The small lycophytes survived, represented today by about 1,200 species. Though some are commonly called club mosses and spike mosses, they are not true mosses
charophytes most closely related to plants
Comparisons of nuclear, chloroplast, and mitochondrial DNA indicate that charophytes in the genera Zygnema and Coleochaete are the closest living relatives of plants ● Note that plants are not descended from modern charophytes, but share a common ancestor with modern charophytes
extant lineages
Extant lineages are those that have surviving members, not only extinct ones
first plants to grow tall
Ferns and other seedless vascular plants vascular tissue allowed these plants to grow tall
Derived Traits of Plants
Five key traits appear in nearly all plants but are absent in the charophytes ● Alternation of generations ● Multicellular, dependent embryos ● Walled spores produced in sporangia ● Multicellular gametangia ● Apical meristems
flagellated sperm
Fla- gellated sperm swim through a film of water toward eggs, entering the archegonia in response to chemical attractants. Eggs are not released but instead remain within the bases of archegonia. After fertilization, embryos are retained within the archegonia. Layers of placental transfer cells help transport nu- trients to the embryos as they develop into sporophytes.
Many bryophytes also reproduce asexually
For example, some mosses produce brood bodies, small plantlets that detach from the parent plant and grow into genetically identical copies of their parent
Multicellular Gametangia
Gametes are produced within organs called gametangia ● Female gametangia, called archegonia, produce a single nonmotile egg ● Male gametangia, called antheridia, produce and release sperm ● Each egg is fertilized within an archegonium
protonema
Germinating moss spores, for ex- ample, characteristically produce a mass of green, branched, one-cell-thick filaments known as a protonema
division of seed plants
Gymnosperms produce seeds that are not enclosed in chambers ● Angiosperms produce seeds that develop inside chambers that originate within flowers- consists of all flowering plants
peristome
In most mosses, the seta becomes elongated, enhancing spore dispersal by elevating the capsule. Typically, the upper part of the capsule features a ring of interlocking, tooth-like structures known as the peristome (see Figure 29.8). These "teeth" open under dry conditions and close again when it is moist. This allows spores to be discharged gradually, via peri- odic gusts of wind that can carry them long distances.
Structure of flagellated sperm.
In species of land plants that have flagellated sperm, the structure of the sperm closely resembles that of charophyte sperm.
Evolution of Roots
Lignified vascular tissue also provides benefits below ground. nstead of the rhizoids seen in bryophytes, roots evolved in the sporophytes of almost all vascular plants. Roots are organs that absorb water and nutrients from the soil. Roots also anchor vas- cular plants, hence allowing the shoot system to grow taller.
Adaptations Enabling the Move to Land
Many species of charophyte algae inhabit shallow waters around the edges of ponds and lakes, where they are subject to occasional drying. natural selection favors individual algae that can survive periods when they are not submerged in water.
Evolution of leaves
Microphylls first appear in the fossil record 410 million years ago, but megaphylls do not emerge until about 370 mil- lion years ago, toward the end of the Devonian period. According to one model of leaf evolution, microphylls originated from sporangia located on the side of the stem Megaphylls, by contrast, may have evolved from a series of branches lying close together on a stem. As one of these branches came to grow above, or overtop, the others, the lower branches became flattened and developed webbing that joined them to one another. These joined branches thus became a leaf attached to the branch that overtopped them (Figure 29.14b). To better understand the origin of leaves, scientists are exploring the genetic control of leaf development.
vascular tissue distinguishes
One way to distinguish groups of plants is whether or not they have an extensive system of vascular tissue, cells joined into tubes that transport water and nutrients throughout the plant body. Most present-day plants have a complex vascular tissue system and are therefore called vascular plants. Plants that do not have an extensive transport system—liverworts, mosses, and hornworts—are described as "nonvascular" plants, even though some mosses do have simple vascular tissue.
grade
Organisms that are grouped based on shared key biological features, rather than shared ancestry, can be referred to as a grade
Formation of a phragmoplast.
Particular details of cell division occur only in land plants and certain charophytes in land plants and certain charophytes, a group of micro- tubules known as the phragmoplast forms between the daughter nuclei of a dividing cell. A cell plate then develops in the middle of the phragmoplast, across the midline of the dividing cell . The cell plate, in turn, gives rise to a new cross wall that separates the daughter cells
two main groups of seedless vascular plants.
Phylum Lycophyta includes club mosses, spike mosses, and quillworts ● Phylum Monilophyta includes ferns, horsetails, and whisk ferns and their relatives
Alternation of Generations
Plants alternate between two multicellular generations, a reproductive cycle called alternation of generations The gametophyte generation is haploid and produces haploid gametes by mitosis Fusion of a sperm and egg gives rise to the diploid sporophyte, which produces haploid spores by meiosis Spores develop into gametophytes
Apical Meristems
Plants sustain continual growth in length by repeated cell division within the apical meristems ● Cells from the apical meristems differentiate into various tissues
bisexual gametophytes ?
Some bryophyte gametophytes are bisexual, but in mosses the archegonia and antheridia are typi- cally carried on separate female and male gametophytes
he Significance of Seedless Vascular Plants
The ancestors of modern lycophytes, horsetails, and ferns grew tall during the Devonian and Carboniferous, forming the first forests ● The decaying plants of these Carboniferous forests formed coal over millions of years
ancestors of vascular plants
The ancestors of vascular plants already had some derived traits of today's vascular plants, but they lacked roots and some other adaptations that evolved later.
Multicellular, Dependent Embryos
The diploid embryo is retained within the tissue of the female gametophyte Nutrients are transferred from parent to embryo through placental transfer cells Plants are called embryophytes because of the dependency of the embryo on the parent
rhizoids
The gametophytes are anchored by delicate rhizoids, which are long, tubular single cells (in liverworts and horn- worts) or filaments of cells (in mosses).
peatlands
The low temperature, pH, and oxygen level of peatlands inhibit decay of moss and other organisms Peatlands cover 3% of Earth's land surface and contain roughly 30% of the world's soil carbon ● Overharvesting of Sphagnum and/or a drop in water level in peatlands could release stored CO2 to the atmosphere
Peroxisome enzymes.
The peroxisomes (see Figure 6.19) of both land plants and charophytes contain enzymes that help minimize the loss of organic products resulting from photorespiration
embryophytes
The placement of the boundary dividing plants from algae is the subject of ongoing debate we define plants as embryophytes, plants with embryos
Walled Spores Produced in Sporangia
The sporophyte produces spores in organs called sporangia ● Diploid cells called sporocytes undergo meiosis to generate haploid spores ● Spore walls contain sporopollenin, which makes them resistant to harsh environments
Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives
Today, ferns are by far the most widespread seedless vascular plants, numbering more than 12,000 species. Though most diverse in the tropics, many ferns thrive in temperate forests, and some species are even adapted to arid habitats.
seedless vascular plants
Two of these clades are the lycophytes (club mosses and their relatives) and the moniolophytes(ferns and their relatives). do not form a clade but a grade g
475-million-year-old rocks
Unlike previously discovered spores of this age, these were embedded in plant cuticle material that is similar to spore-bearing tissue in living plants (Figure 29.6b). After uncovering other small fragments of tissue that clearly belonged to plants, the scientists concluded that the spores from Oman represent fossil plants rather than algae. Whatever the precise age of the first land plants, those an- cestral species gave rise to the vast diversity of living plants.
difference between rhizoids and roots
Unlike roots, which are found in vascular plant sporophytes, rhizoids are not composed of tissues. Bryophyte rhizoids also lack specialized conducting cells and do not play a primary role in water and mineral absorption.
Origins and Traits of seedless Vascular Plants
Unlike the nonvascular plants, these species had branched sporophytes that were not de- pendent on gametophytes for nutrition their branching made possible more complex bodies with multiple sporangia.
Transport in Xylem and Phloem
Vascular plants have two types of vascular tissue: xylem and phloem.
what percent of plant are vascular
Vascular plants, which form a clade that comprises about 93% of all extant plant species, can be categorized further into smaller clades.
The Ecological and Economic Importance of Mosses
Wind dispersal of lightweight spores has distributed mosses throughout the world. These plants are particularly common and diverse in moist forests and wetlands. Some mosses that colonize bare, sandy soil help to reduce nitrogen loss from the soil; others harbor nitrogen-fixing cyanobacteria
Xylem
Xylem conducts most of the water and mineral The xylem of most vascular plants includes tracheids, tube- shaped cells that carry water and minerals up from roots Water-conducting cells are strengthened by lignin and provide structural support (polymer)
struggles of charophytes moving to land
a scarcity of water and lack of structural support against gravity Plants diversified as adaptations evolved that enabled them to thrive on land despite challenges
seed
an embryo packaged with a supply of nutrients inside a protective coat.
nearly 90 percent of all plant species are
angiosperms
Leaves
are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis
The term Charophyte
came froms the Latin chara , meaning"an unknown type of root" and-phyta, meaning "plant
additional derived traits in plants
cuticle - a waxy covering of the epidermis stomata- specialized cells that allow for gas exchange between the outside air and the plant Mycorrhizae- symbiotic associations between fungi and plants may have helped plants without true roots obtain nutrients
Bryophyte sporophytes consists of
foot- Embedded in the archegonium, the foot absorbs nutrients from the gametophyte. seta (stalk) - conducts these materials to the sporangium, also called a capsule, which uses them to produce spores by meiosis. and a sporangium, also called a capsule- which discharges spores through a peristome
Unlike vascular plants, in all three bryophyte phyla
gametophytes (haploid) are larger and longer-living than sporophytes When bryophyte spores are dispersed to a favorable habi- tat, such as moist soil or tree bark, they may germinate and grow into gametophytes. Sporophytes are typically present only part of the time
plants evolved from
green algae called charophytes
• Rings of cellulose-synthesizing proteins.
he cells of both land plants and charophytes have distinctive circular rings of proteins in the plasma membrane (Figure 29.2). These protein rings synthesize the cellulose microfibrils of the cell wall. In contrast, noncharophyte algae have linear sets of proteins that synthesize cellulose.
Life Cycles with Dominant Sporophytes
he sporophyte (diploid) generation is the larger and more complex plant in the alternation of generations
phloem
he tis- sue called phloem has cells arranged into tubes that dis- tribute sugars, amino acids, and other organic products
All seed plants and a few seedless vascular plants are
heterosporous.
sporophylls,
modified leaves that bear sporangia.
sporopollenin
n charophytes, a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out. A similar chemical adaptation is found in the tough sporopollenin walls that encase the spores of plants. allowed for the movement to land
Sphagnum,
or "peat moss," forms extensive deposits of partially decayed organic material known as peat Peat can be used as a source of fuel The low temperature, pH, and oxygen level of peatlands inhibit decay of moss and other organisms
other characteristics of pants shared with general algae
plants are multicellular, eukaryotic, photosynthetic autotrophs, as are brown, red, and certain green algae. lants have cell walls made of cellulose, and so do green algae, dinoflagellates, and brown algae. chloroplasts with chlorophylls a and b are present in green algae, euglenids, and a few dinoflagellates, as well as in plants.
A third clade of vascular plants consists of
seed plants
ferns and other pterophytes are more closely related to
seed plants than to lycophytes. As a result, pterophytes and seed plants share traits that are not found in lycophytes, including overtopping growth (see Figure 29.14b), megaphyll leaves, and roots that can branch at various points along the length of an existing root In lycophytes by contrast, roots branch only at the growing tip of the root, forming a Y-shaped structure.
diversity of plants
since colonizing land, plans have diversified into more than 290,000 living species most live on land, a few live in aquatic habitats
Sporophylls vary greatly in structure.
sori - are clusters of sporangia on the undersides of sporophylls in fern Strobili- Strobili are cone-like structures formed from groups of sporophylls In many lycophytes and in most gymnosperms
difference in fossil spores and modern spores
spores of present-day plants are typically dispersed as single grains, but the fossil spores are fused together in groups of two or four - fossil spores could actually be from an algae relative Furthermore, the oldest known fragments of plant body tissues are 50 million years younger than the puzzling spores. ● Fossils of larger structures, such as a sporangium, date to 425 million years ago
One milestone in the evolution of plants was the emergence of
sporophylls,
are algae plants ?
● Algae are not included in the plant kingdom; they are photosynthetic protists
The nonvascular plants (bryophytes) are represented today by three phyla of small herbaceous (nonwoody) plants:
● Liverworts, phylum Hepatophyta ● Mosses, phylum Bryophyta ● Hornworts, phylum Anthocerophyta ● These groups are thought to represent the earliest lineages to diverge from the common ancestor of land plants
Leaves are categorized by two types:
● Microphylls, small, often spine-shaped leaves with a single vein ● Megaphylls, larger leaves with a highly branched vascular system
orphological and Molecular Evidence that plants evolve from green algae
● Rings of cellulose-synthesizing proteins ● Structure of flagellated sperm ● Formation of a phragmoplast -peroxisome enzymes
Origin and diversification of plants
● The appearance of plant spores in the fossil record indicates that plants colonized land at least 470 million years ago ● Fossilized spores and plant tissues have been extracted from 450-million-year-old rocks