Plant DiversityEcol 182 – 3-1-2007

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Summary from last time

• We talked about?

The Tracheophytes

• A leaf is a flattened photosynthetic structure emerging laterally from a main axis or stem and possessing true vascular tissue.

• Two leaf types: microphylls and megaphylls.• The microphyll has a single vascular strand that has

departed from the stem without disturbing the stem’s vascular structure. – Club mosses have microphylls.– Microphylls may have evolved from sterile sporangia.

Figure 29.13a The Evolution of Leaves

The Tracheophytes• The megaphyll is larger, and more complex found in

ferns and seed plants.• May have arose from flattening of stems and

development of overtopping (one branch differentiates from and extends beyond rest).

Introducing the Tracheophytes• Plants that bear a single type of spore are said to

be homosporous.

– The most ancient tracheophytes were all homosporous.– Both the gametophyte and the sporophyte are

independent and usually photosynthetic.– A single type of gametophyte bears both female and

male reproductive organs.

Introducing the Tracheophytes• Plants with two distinct types of spores evolved

later, and are said to be heterosporous. – In heterosporous plants, the megaspore develops into a

larger, specifically female gametophyte (megagametophyte).

– The microspore develops into the smaller, male gametophyte (microgametophyte).

• Heterospory evolved independently and repeatedly, suggesting that it affords selective advantages.

Figure 29.14a & b Homospory and Heterospory

The Surviving Nonseed Tracheophytes

• The club mosses (phylum Lycophyta) have microphylls, exhibit apical growth, and have roots that branch dichotomously.

• Sporangia in many Lycophyta are contained within structures called strobili (clusters of spore-bearing leaves) – There are both homosporous and heterosporous

species.– Lycophyta and the Pteridophyta were the dominant

during the Carboniferous period.

Figure 29.15 Club Mosses

The Surviving Nonseed Tracheophytes

• The horsetails, whisk ferns, and ferns form a clade, the phylum Pteridophyta.

• The leaves are reduced megaphylls and grow in whorls.

• Stem growth is from the base of the stem segments.

Figure 29.16 Horsetails

The Surviving Nonseed Tracheophytes• The sporophytes of the ferns typically have true

roots, stems, and leaves.• The ferns first appeared during the Devonian.• Ferns are characterized by fronds, large leaves with

complex vasculature.• Sporangia are found on the undersurfaces of the

fronds, clustered in groups called sori.

Figure 29.19 Fern Sori Are Clusters of Sporangia

Figure 29.20 The Life Cycle of a Fern

Figure 29.10 The Evolution of Today’s Plants

• Seed plants are the most derived tracheophytes.– Gymnosperms (such as pines and cycads) – four phyla– Angiosperms (flowering plants) – one phyla

• Big evolutionary innovations– Evolution of a ‘seed’– Reduction in gametophyte generation

• The haploid gametophyte is attached to and nutritionally dependent on the diploid sporophyte.

The Seed Plants

Figure 30.2 The Relationship between Sporophyte and Gametophyte Has Evolved (Part 1)

The Seed Plants• The seed plants are heterosporous

– Separate megasporangia and microsporangia

• Megaspores produce a single, haploid, multicellular female gametophyte in megasporangia

• Microspores meiotically divide to produce pollen grains in microsporangia

– Fertilization occurs through pollen tube elongation to the female gametophyte (which release two sperm)

• Resulting zygote divides until an embryonic stage is reached, when growth is halted (producing a seed).

The Seed Plants• A seed may contain tissues from three generations.

– Seed coat and megasporangium develop from the diploid sporophyte parent.

– In the megasporangium, the haploid female gametophyte tissue is of the next generation.

– The center of the seed contains a third generation, the embryo of the new diploid sporophyte.

• The possession of seeds is a major reason for the enormous evolutionary success of seed plants.

The Gymnosperms: Naked Seeds

• The gymnosperms do not produce flowers, and their ovules and seeds are not protected by flower or fruit tissue.

• There are four clades of living gymnosperms today.

– Phylum Cycadophyta, the cycads

– Phylum Ginkgophyta has a single species, Ginkgo biloba.

– Phylum Gnetophyta

– Phylum Pinophyta, the conifers

The Gymnosperms: Naked Seeds• Fir, cedar, spruce, and pine all belong to Pinophyta

– Megaspores are produced in cones– Microspores are produced in pollen strobili

•About ½ of conifers have fruit-like tissues surrounding seeds that are eaten by animals resulting in dispersal in their feces (but they are not true fruits)

Figure 30.6 The Life Cycle of a Pine Tree

The Gymnosperms: Naked Seeds• Gymnosperms exhibit secondary growth

• Recall types of types of growth (animals versus plants)– Determinate - body ceases to grow once adulthood is reached.– Indeterminate – ‘body’ growth is potentially continuous

• Meristematic regions are localized regions of cell division.– They produce new cells indefinitely

• When meristem cells divide, one daughter cell develops into another meristem cell; the other develops specialization.

• Two meristem types:

– Apical meristems give rise to the primary plant body.

– Lateral meristems give rise to the secondary plant body.• Lateral meristems give rise to tissues responsible for stems and roots

thickening to form wood.

Figure 35.13 Apical and Lateral Meristems

Figure 35.15 Tissues and Regions of the Root Tip

Forming the Plant Body• Secondary tissues derive from two lateral meristems: vascular and

cork cambium.– Vascular cambium - a cylindrical tissue that form the secondary xylem,

and the secondary phloem– Cork cambium - produces the outermost layers of stems protecting tissues

from H2O loss & microorganisms.

• Growth in the diameter of the stems and roots, produced by thesemeristematic regions is called secondary growth.

– Wood is secondary xylem.– Bark is everything produced external to the vascular cambium (including

secondary phloem).

Figure 35.14 A Woody Twig

Gymnosperms: Naked Seeds

• Gymnosperms (except Gnetophyta) have only tracheids, and simple phloem.

• Tracheids are simple xylem that conduct water throughout the plant body

– Tracheids undergo apoptosis and operate as empty cells (cell walls remain).

• Phloem are alive, and transport carbohydrates and other materials throughout the plant

The Angiosperms: Flowering Plants

• Phylum Angiospermae ~ 257,000 species.– Angiosperm means “enclosed seed.”

• The angiosperms are the most derived form of the tracheophytes

– the sporophyte generation is larger and has greater independence from the gametophyte

– the gametophyte is smaller and more dependent on the sporophyte

The Angiosperms: Flowering Plants

• A number of synapomorphies, or shared derived traits, characterize the angiosperms:

– They have double fertilization (upcoming figure).

– They produce triploid endosperm.

– Their ovules and seeds are enclosed in a carpel (modified leaf).

– They have flowers (modified leaves).

– They produce fruit (at minimum – mature ovary and seed).

– Their xylem contains vessel elements (specialized H2O transport) and fibers (structural integrity).

– Their phloem contains companion cells (assists with metabolic issues associated with transport).

The Angiosperms: Flowering Plants

• Double fertilization - two male gametes participate in fertilization events within the megagametophyte.

– One sperm combines with the egg to produce a diploid zygote.

– The other sperm combines with two other haploid nuclei of the female gametophyte to form a triploid nucleus

• Results in endosperm - tissue that nourishes the embryonic sporophyte

Figure 30.11 The Life Cycle of an Angiosperm

The Angiosperms: Flowering Plants• All the parts of a flower are

modified leaves.

• Stamens - filament bearing anthers containing pollen-producing microsporangia.

• Pistil – one or more carpels with a swollen base (ovary) containing megasporangia.

• Style is the apical stalk of the pistil (terminal surface receiving pollen is called the stigma)

The Angiosperms: Flowering Plants

• Specialized leaves (petals and sepals) are important for attracting pollinators

– Many angiosperms are animal-pollinated increasing the likelihood of outcrossing (in exchange for nectar or pollen)

– Coevolution has resulted in some highly specific interactions, but most plant-pollinator systems are not highly specific

• Evolutionarily ancient angiosperms have a large and variable number of floral structures (petals, sepals, carpels, and stamens)

– Evolutionary trend within the group: reduction in number of floral organs, differentiation of petals and sepals, changes in symmetry, and fusion of parts.

Figure 30.8 Inflorescences

The Angiosperms: Flowering Plants

• Perfect flowers have both microsporangia and megasporangia.

• Imperfect flowers (have either, but not both).– Monoecious species produce both types of imperfect flowers on the

same plant.– In dioecious species, a plant produces either megasporangiate or

microsporangiate flowers but not both.

• Developing embryos consists of an embryonic axis and one or two cotyledons (seed leaves), which metabolize endosperm and may become photosynthetic.

Figure 35.1 Monocots versus Eudicots

Figure 35.2 Vegetative Organs and Systems

Organs of the Angiosperms• Two main types of root system: taproot and fibrous root.

• Many eudicots have a taproot system: a single, large, deep-growing primary root with smaller lateral roots.

• Monocots and some eudicots have a fibrous root systemcomposed of numerous thin roots roughly equal in diameter.

– A fibrous root system holds soil in place very effectively.

• Some plants have adventitious roots, which arise from points along the stem where roots would not usually occur.

Figure 35.3 Root Systems

Angiosperm vascular systems

• Xylem in angiosperms consists of vessel elements in addition to tracheids

– Vessel elements also conduct water and are formed from dead cells.

– Vessel elements are generally larger in diameter than tracheids and are laid down end-to-end to form hollow tubes.

• Sieve tube elements (Phloem) in Angiosperms are stacked, similar to xylem

– Have adjacent companion cells that retain all organelles

– Companion cells may regulate the performance of the sieve tube members through their effects on active transport of solutes

Figure 35.9 Plant Cell Types (Part 3)

Why is a greater diameter a big deal for the evolution of plants?

Figure 35.10 Evolution of the Conducting Cells of Vascular Systems

Figure 35.11 Sieve Tubes

Angiosperms: Flowering Plants• Monocots - a single embryonic

cotyledon (grasses, cattails, lilies, orchids, and palms)

• Eudicots - two cotyledons, and include the majority of familiar seed plants

• Additional clades - water lilies, star anise, and the magnoliid complex

• Big question in plant evolution –what is the basal angiosperm?