Evolution and Diversity in Plants II - Ecol 182 – 4-12-2005

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Figure 29.4 From Green Algae to 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 (modified stem bearing a tight cluster of scales specialized for reproduction)

– Microspores are produced in pollen strobili (a conelike cluster of scales that are modified leaves)

•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 these meristematic 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 system composed 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?

Plant Structure and Function I - Ecol 182 – 4-12-2005

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• Uptake and Movement of Water and Solutes

• Transport of Water and Minerals in the Xylem

• Transpiration and the Stomata

• Translocation of Substances in the Phloem

General problem in plant function• Need for H2O for:

– photosynthesis, – Solute transport,– temperature control,– internal pressure for growth

• Plants obtain water and minerals from the soil via the roots

– in turn roots extract carbohydrates and other important materials from the leaves.

• Water enters the plant through osmosis

– but the uptake of minerals requires transport proteins.

Uptake & Movement of Water & Solutes in Plants

• Osmosis is the diffusion of water through a membrane – primary means of water transport in plants

• Osmotic potential, or solute potential, determines the direction of water movement across a membrane.

– Potential refers to the potential energy contained in the system measured

– Dissolved solutes have the effect of lowering the concentration of water (changing the potential energy).

– Greater solute concentration results in a more negative solute potential and a greater the tendency of water to diffuse to the solution.

Uptake & Movement of Water & Solutes in Plants

• Water potential is the tendency of a solution to take up water from pure water ().

• Water potential of a system is the sum of the negative solute potential (s) and the (usually positive) pressure potential (p).

s + p

• Solute potential, pressure potential, and water potential are measured in megapascals (Mpa).

Figure 5.8 Osmosis Modifies the Shapes of Cells

Figure 36.2 Water Potential, Solute Potential, and Pressure Potential

Figure 36.4 Apoplast and Symplast

Figure 36.5 Casparian Strips

Transport of Water and Minerals in the Xylem

• The adhesion-cohesion–tension theory of water movement:

• The concentration of water vapor is higher inside the leaf than outside, so water diffuses out of the leaf through the stomata (this is transpiration).

• This creates a tension in the mesophyll that draws water from the xylem of the nearest vein into the apoplast surrounding the mesophyll cells

• The removal of water from the veins, in turn, establishes tension on the entire volume of water in the xylem, so the column is drawn up from the roots.

Figure 36.8 The Transpiration–Cohesion–Tension Mechanism

Transport of Water and Minerals in the Xylem

• Hydrogen bonding – results in cohesion (sticking of molecules to one another).

• The narrower the tube, the greater the tension the water column can stand.

• Maintenance of the water column also occurs through adhesion of water molecules to the walls of the tube.

Transport of Water and Minerals in the Xylem

• The key elements in water transport in xylem:

– Transpiration

– Tension

– Cohesion

• The transpiration–cohesion–tension mechanism does not require energy.

• At each step, water moves passively toward a region with a more negative water potential.

Transport of Water and Minerals in the Xylem

• Mineral ions in the xylem sap rise passively with the solution.

• Transpiration also contributes to the plant’s temperature regulation, cooling plants in hot environments.