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AP Biology Notes Outline
Plant Diversity I: How Plants Colonized Land
Chapter 29
OVERVIEW: Plants are defined as multicelled, eukaryotic, photosynthetic autotrophs. Their cell walls are
made of cellulose, and their surplus carbohydrate is stored as starch. The life cycle of plants is characterized by
alternation of generations. One generation is the gametophyte generation, where all the cells of the plant body
are haploid (n). The other generation is the sporophyte generation, where all the cells of the plant body are
diploid (2n). The plant kingdom is divided into divisions, instead of phyla. Biologists believe that modern
multicelled plants evolved from green algae.
Table 29.1
The FOUR main groups of land plants are:
1. Bryophytes: non-vascular plants
• mosses, hornworts, and liverworts
2. Pteridophytes: seedless vascular plants
• Club moss, horsetails, ferns
3. Gymnosperms: vascular seeded cone-bearers
• Ginkgos, cycads, gnetophytes, conifers
4. Angiosperms: vascular seeded flowering plants
• Monocots & dicots
FIGURE 29.5 – Evolutionary adaptations to terrestrial living characterize the four main groups of land plants.
The four main plant groups are bryophytes (including mosses), pteridophytes (seedless vascular plants,
including ferns, gymnosperms (cone-bearers, including conifers), and angiosperms (flowering plants).
FOUR major evolutionary trends characterize plants today:
• Movement to land
• Development of vascular tissue
• Development of seeds
• Development of flowers
Land plants evolved from charophycean (green) algae over 500 million years ago. There are four key traits that land plants share only
with charophyceans:
1. Rose-shaped complexes for cellulose synthesis
2. Peroxisome enzymes
3. Structure of flagellated sperm
4. Formation of a cell plate during cell division
5. Genetic similarities including nuclear and chloroplast analysis
FIGURE 29.5 – KNOW IT!
Five key traits appear in nearly all land plants but are absent in the charophyceans:
1. Apical meristems (localized regions of cell division at tips of roots & shoots) – maximize exposure to environmental
resources
2. Alternation of generations – characteristic life cycle of all plants (diploid & haploid phase alternate)
3. Walled spores produced in sporangia (plant spores are haploid reproductive cells – the walls of plant spores contains
sporopollenin – durable material that makes plant spores very tough and resistant to harsh environments – make possible air
dispersion without drying out)
4. Multicellular gametangia (found in bryophytes, pteridophytes, and gymnosperms – this is where gametes are produced
within the plant) female gametangia is archegonia and male gametangia is antheridia
5. Multicellular dependent embryos (develop from zygotes that are retained within tissues of female parent – parental tissues
provide developing embryo with nutrients)
LIFE CYCLE OF TYPICAL ANGIOSPERM:
sporophyte→meiosis→gametophyte→gametes→fertilization→zygote→sporophyte
AP Biology Notes Outline
Plant Diversity I: How Plants Colonized Land
Chapter 29
FIGURE 29.5 – The sexual life cycle in plants is characterized by alternation of generations in which the
haploid (n) and the diploid (2n) generations alternate with each other.
The gametophyte (n) produces gametes by mitosis. These gametes fuse during
fertilization to yield (2n) zygotes. Each zygote develops into a sporophyte (2n),
which produces haploid spores (n) by meiosis. Each haploid spore forms a new
gametophyte, completing the life cycle.
Plant spores are haploid reproductive cells that have the potential to grow into
multicellular, haploid gametophytes by mitosis.
• A polymer called sporopollenin (durable organic material) makes walls
of plant spores tough and resistant to harsh environments.
• Multicellular organs called sporangia, found on the sporophyte
generation of a plant, produce the spores.
• The outer tissues of the sporangium protect developing spores until they
are ready to be released – a key adaptation in land plants.
The gametophyte forms of bryophytes, pteridophytes, and gymnosperms all
produce their gametes within multicellular organs called gametangia.
• Female gametangia are called archegonia
• Male gametangia are called antheridia
• Multicellular plant embryos develop from zygotes that are retained within tissues of the female parent.
FIGURES 29.5 – Plants began life in the seas and moved to land as competition for resources increased. The
biggest problems a plant on land faces are supporting the plant body and absorbing and conserving water. The
following adaptations are strategies that enabled plants to move to land.
1.
Cell walls made of cellulose lend support to the plant whose cells, unsupported by a watery environment, must maintain their own
shape.
2.
Roots and root hairs maximize contact with environmental resources by absorbing water and nutrients from the soil.
3.
Except for bryophytes, land plants have true roots, stems, and leaves which are defined by the presence of vascular tissues (xylem
and phloem).
4.
Stomates open to exchange photosynthetic gases and close to minimize excessive water loss.
5.
Stomatal crypts further reduce water loss.
6.
The waxy coating of the leaves, the cuticle, helps prevent excess water loss from the leaves.
7.
Plants have multicellular, dependent embryos that are retained within tissues of the female parent. The parental tissue provides
the developing embryo with nutrients.
8.
In some plants, gametes and zygotes form within a protective jacket of cells called gametangia, which prevents them from drying
out.
9.
Sporopollenin, a tough polymer, is resistant to almost all kinds of environmental damage and protects plants in a harsh terrestrial
environment. It is found in the walls of spores and pollen.
AP Biology Notes Outline
Plant Diversity I: How Plants Colonized Land
Chapter 29
10. Because plants cannot move, the elongation and branching of their shoots and roots maximize their exposure to environmental
resources. This is sustained throughout the life of the plant by the activity of apical meristems (localized regions of cell division
and the tips of shoots and roots).
11. C4 and CAM plants can survive in dry environments.
12. Seeds and pollen have a protective coat that prevents desiccation. They are also a means of dispersing offspring over a wide
range. Flowers and fruits also aid in seed disperal.
13. The gametophyte generation has been reduced.
FIGURES 29.8 & 29.9 – NON-VASCULAR PLANTS: Bryophytes are NONVASCULAR PLANTS that live
in moist environments and absorb water by diffusion. They lack lignin-fortified tissue required to support tall
plants on land, therefore they grow low to the ground in moist environments. The three phyla of bryophytes are
mosses, liverworts, and hornworts.
The alternation of generation life cycles of mosses and
other bryophytes are dominated by the gametophyte stage.
Bryophytes are represented today by three phyla of small
herbaceous (nonwoody) plants:
• Liverworts, phylum Hepatophyta
• Hornworts, phylum Anthocerophyta
• Mosses, phylum Bryophyta
In all three bryophyte phyla
• Gametophytes are larger and longer-living than
sporophytes
• Water is required for survival AND reproduction
– absorb water by diffusion (flagellated sperm
required a film of water to reach eggs).
• Have no true roots, stems, and leaves and lack
lignin-fortified tissue required to support tall
plants on land – therefore, these plants grow low
to the ground and are typically found in moist
environments!
Life Cycle of Moss – Figure 29.8:
1) Most species of moss have separate male and female gametophytes, which have antheridia and archegonia.
2) A FLAGELLATED sperm SWIMS through a film of moisture to an archegonium and fertilizes the egg.
3) The diploid zygote divides by mitosis and developes into an embryonic sporophyte within the archegonium.
4) During the next stage of its development, the sporophyte grows a long stalk that emerges from the archegonium.
5) The base of the sporophyte REMAINS ATTACHED TO AND NUTRITIONALLY DEPENDENT on the female
gametophyte.
6) The sporangium is at the tip of the stalk, a capsule in which meiosis occurs and haploid spores develop. The spores scatter
when the lid of the sporangium pops off.
7) Spores germinate by mitotic division, forming small, green, threadlike protonemata.
8) These protonemata continue to grow, eventually forming sexually mature gametophytes, completing the life cycle.
AP Biology Notes Outline
Plant Diversity I: How Plants Colonized Land
Chapter 29
FIGURES 29.13 & 29.15 – VASCULAR PLANTS: Additional terrestrial adaptations evolved as vascular
plants descended from moss-like ancestors. Tracheophytes have transport vessels: xylem and phloem. This
group includes seedless vascular plants (ferns & their relatives) that reproduce by spores. It also includes
modern plants that reproduce by seeds (gymnosperms and angiosperms).
Tracheophytes are the most common and widespread of land plants. They have TWO TYPES of true vascular tissues (xylem and
phloem) that allow true “organs” to develop – roots, stems, leaves
• Xylem carries water and minerals in a plant (includes dead cells called tracheids)
• Phloem carries nutrients in a plant (consists of living cells)
The two categories of vascular plants are (1) Seedless - reproduce by spores and (2) Seeded - reproduce by seeds
In contrast with bryophytes:
• Sporophytes of seedless vascular plants are the larger generation, as in the familiar leafy fern
• The gametophytes are tiny plants that grow on or below the soil surface
Roots are organs that anchor vascular plants. They enable vascular plants to absorb water and nutrients from the soil. Leaves are
organs that increase the surface area of vascular plants, thereby capturing more solar energy for photosynthesis
• Leaves are categorized by two types (Microphylls, leaves with a single vein) and (Megaphylls, leaves with a highly branched
vascular system) – FIGURE 29.14.
• Sporophylls are modified leaves with sporangia (TEXT PAGE 612 & 613).
• Most seedless vascular plants are homosporous, producing one type of spore that develops into a bisexual gametophyte. All
seed plants and some seedless vascular plants are heterosporous, having two types of spores that give rise to male and female
gametophytes –TEXT PG 613.
FIGURES 29.15 – SEEDLESS VASCULAR PLANTS: Lycophyta and Pterophyta are the two phyla of
modern deedless vascular plants. The growth of these early forests may have helped produce the major global
cooling that characterized the end of the Carboniferous period - decayed and eventually became coal.
Seedless vascular plants form two phyla:
• Lycophyta, including club mosses, spike mosses, and quillworts
• Pterophyta, including ferns, horsetails, and whisk ferns and their relatives
A sporophyte-dominant life cycle evolved in seedless vascular plants. The ancestral condition of flagellated sperm is retained by all
seedless vascular plants. Since they don’t have seeds, they need LOTS of water for fertilization
• Display alternation of generations – part of time is asexual (uses spores), part is sexual (uses gametes)
• USE SPORES TO REPRODUCE FOR MAJORITY OF LIFE; gametes are used only for a short period of time
• Note Figure 29.13 – Life Cycle of a Fern!