Lecture 8 Notes
1. The Four Floral Whorls (outside → inside)
A typical flower has four concentric layers (whorls):
Sepals (Calyx)
Protective layer when the flower is a bud
Often green because they are not primarily for attraction
Can be:
Free sepals (separate units)
Fused sepals (form a tube)
Petals (Corolla)
Usually colourful for pollinator attraction
Can also be:
Free petals
Fused petals (tube)
Stamens (male reproductive structure)
Made of:
Filament (support)
Anther (pollen production)
Carpel / Pistil (female reproductive structure)
Made of:
Stigma (receives pollen)
Style (tube pollen grows through)
Ovary (contains ovules)
Ovule (contains 1 egg cell)
2. Structure of the Anther
Each anther typically has 4 pollen sacs
Pollen develops and matures inside these sacs
When pollen is ready:
The thin walls between adjacent sacs break down
A slit forms
Pollen is released through this slit
So one slit releases pollen from two sacs.
3. The Stigma and Style (pollen guidance system)
Stigma
Surface is not flat
Has specialized epidermal cells:
Conical shapes
Hair-like structures
These act like a lock-and-key system to trap pollen
Increases chance of pollen germination
Style
A hollow tube
Inner surface produces:
Slippery substances
Nutrients for pollen tube growth
Pollen tube grows:
stigma → through style → to ovary
4. Ovary, Ovule, Egg Cell
Very important distinction:
Ovary can contain one or many ovules
Each ovule has only one egg cell
Examples shown:
Plant | Ovary type |
|---|---|
Cherry, peach, olive, almond | One ovule → one seed (pit) |
Blackberry, raspberry | Many free carpels, each with one ovule |
Cucumber, zucchini | Fused carpels, many ovules per ovary |
5. Carpel vs Pistil (critical concept)
Definition
A carpel = stigma + style + ovary
When are they the same?
If the flower has only one carpel → 1 carpel = 1 pistil
If the flower has multiple free carpels → number of pistils = number of carpels
When are they different?
As soon as carpels are fused:
There may be 3 carpels
But they form 1 pistil
Rule:
Fused carpels → pistil number does NOT equal carpel number
Example:
3 fused carpels forming one ovary → 1 pistil made of 3 carpels
6. Flower → Fruit
After fertilization:
Ovary enlarges → fruit
Ovules → seeds
Sepals, petals, stamens fall off
Remnants sometimes visible (e.g., bottom of pear)
7. Types of Flowers Based on Sex
A. Complete (Hermaphrodite) Flowers — 75%
Have both stamens and pistil in the same flower
Examples: trillium, raspberry
B. Monoecious Plants — 17%
Separate male and female flowers
On the same plant
Examples:
Corn:
Tassel = male flowers
Ear = female flowers
Hazelnut (catkins are male)
C. Dioecious Plants — 6%
Male and female flowers on different plants
Examples:
Cannabis
Asparagus
This is like animals.
8. Selfing vs Outcrossing
Selfing
Fertilization using pollen from the same genetic individual.
Leads to:
Inbreeding depression
Weak offspring
Outcrossing
Different individuals.
Leads to:
Genetic diversity
Vigorous plants
Plants prefer outcrossing but keep selfing as backup.
9. Why Hermaphrodite is Most Common
Plants are immobile and rely on pollinators.
If no pollinator is present:
Hermaphrodite flower can self-fertilize
Produces seeds rather than none
Better poor offspring than no offspring.
10. How Plants Avoid Selfing (very testable)
Three mechanisms:
Temporal separation
Spatial separation
Self-incompatibility (mentioned but for later lecture)
11. Temporal Separation (different timing of male/female function)
Protogyny (female first)
Stigma receptive
Anthers closed
Later:
Stigma shuts down
Anthers open
Protandry (male first)
Anthers release pollen
Stigma closed
Later:
Stigma opens
Anthers empty
Problem:
If plant has many flowers (e.g., avocado), different flowers may be in different phases → cross-flower selfing still possible.
12. Spatial Separation (different positions)
Two versions:
Long style → stigma above anthers
Short style → stigma below anthers
Creates physical distance, reducing selfing.
13. Special Case: Primrose (Heterostyly)
Species produces two flower morphs:
Morph | Style | Anthers |
|---|---|---|
Morph 1 | Short | High |
Morph 2 | Long | Low |
Important rules:
One plant produces only one morph
Pollen attaches to a specific part of insect
That position matches the stigma of the opposite morph
Result:
Efficient cross-pollination
Reduced selfing within same morph
14. Reproductive Systems and Selfing Possibility
System | Within-flower selfing | Cross-flower selfing | Outcrossing |
|---|---|---|---|
Dioecious | No | No | Yes only |
Monoecious | No | Yes | Yes |
Hermaphrodite | Yes | Yes | Yes |
15. Fruit Structure Examples
Cherry / Peach / Olive
1 carpel
1 ovule
1 seed (pit)
Blackberry / Raspberry
Many free carpels
Each becomes a small fruit unit
Cucumber / Zucchini
3 fused carpels
Many ovules → many seeds
16. Key Takeaway Concepts You Must Know
You must be able to explain:
Free vs fused sepals/petals
Stamen structure and pollen release
Stigma and style adaptations
Ovary vs ovule vs egg cell
Carpel vs pistil rule
Differences between cherry, blackberry, cucumber ovaries
Hermaphrodite vs monoecious vs dioecious
Why selfing is bad but useful
Temporal vs spatial separation
Primrose heterostyly example and how it works
Primrose heterostyly exemplifies how plants can promote genetic diversity by having different floral morphs that encourage cross-pollination. In this system, primroses have two forms: pin flowers, which have long styles and short stamens, and thrum flowers, which have short styles and long stamens. This spatial separation of reproductive organs helps to minimize self-fertilization, as pollen from one flower type is better suited to fertilize the other, thus enhancing genetic variations within the population.
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1. The Four Floral Whorls (outside
A typical flower is composed of four concentric layers of modified leaves, known as whorls, ordered from outermost to innermost:
Sepals (Calyx)
The outermost whorl, primarily functioning as a protective casing for the flower bud before it opens.
Typically green and photosynthetically active, as they are not designed for pollinator attraction.
Morphology:
Free sepals (Apopetalous structure: units are separate).
Fused sepals (Sympetalous structure: units are joined, forming a calyx tube).
Petals (Corolla)
Located just inside the sepals, these are usually brightly coloured or scented to serve as the primary visual and olfactory signals for pollinator attraction.
In some species, petals may contain specialized markings (e.g., nectar guides, often visible only under UV light) directing pollinators to the nectaries.
Morphology:
Free petals (separate units).
Fused petals (forming a corolla tube, common in species like petunias).
**Stamens (Androecium
Together, all stamens form the Androecium. This is the male fertile whorl, responsible for producing pollen.
Structure includes two main parts:
Filament: The stalk that supports the anther, holding it in a position accessible to pollinators.
Anther: The terminal structure where pollen grains (the male gametophytes) are developed and matured.
**Carpel / Pistil (Gynoecium
Together, all carpels or pistils form the Gynoecium, the innermost, female fertile whorl.
A single carpel is structurally defined by three primary regions:
Stigma: The receptive surface where pollen lands and germinates.
Style: The stalk connecting the stigma to the ovary; it acts as a pathway for the growing pollen tube.
Ovary: The enlarged basal portion that contains one or more ovules.
Ovule: Structure within the ovary containing the megasporangium and, critically, one egg cell (part of the female gametophyte/embryo sac).
2. Structure of the Anther and Pollen Release
Each anther lobe typically contains two theca (compartments), resulting in a total of 4 microsporangia (often referred to as pollen sacs) within the anther.
Pollen develops from microspores inside these sacs.
When pollen is fully mature and ready for dispersal, the process called dehiscence occurs:
Enzymes break down the thin tissue separating the adjacent pollen sacs (microsporangia).
A specialized slit (or stomium) forms, usually longitudinally along the anther.
This slit releases pollen simultaneously from the two fused pollen sacs, allowing for mass dissemination.
3. The Stigma and Style (Pollen Guidance System)
Stigma (A Receptive Landing Pad)
The stigma surface is highly specialized and seldom flat.
It often possesses unique epidermal cells designed to capture and anchor specific pollen grains:
Conical shapes (papillae)
Hair-like structures (trichomes)
This complex morphology enhances the likelihood that compatible pollen will adhere and germinate, acting like an initial lock-and-key system for species recognition.
Style (The Pollen Tube Pathway)
The style is not merely an inert connection; it contains the transmitting tissue.
This inner tissue secretes essential compounds:
Slippery substances (mucilage) to facilitate tube movement.
Nutrients (sugars and proteins) critical for fueling the rapid growth of the pollen tube.
The ultimate objective of the pollen tube is to navigate the distance: **stigma
4. Ovary, Ovule, Egg Cell (Precise Terminology)
It is crucial to understand the hierarchy of the female reproductive structures:
Ovary: The entire structure can contain one or many ovules (often determined by species, e.g., cherries have one, watermelons have hundreds).
Ovule: Each ovule houses the embryo sac (female gametophyte) and contains only one haploid egg cell ready for fertilization.
5. Carpel vs Pistil (critical concept)
Definition
A carpel = stigma + style + ovary
When are they the same?
If the flower has only one carpel
If the flower has multiple free carpels
When are they different?
As soon as carpels are fused:
There may be 3 carpels
But they form 1 pistil
Rule:
Fused carpels does NOT equal carpel number
Example:
3 fused carpels forming one ovary
6. Flower
After fertilization:
Ovary enlarges
Ovules
Sepals, petals, stamens fall off
Remnants sometimes visible (e.g., bottom of pear)
7. Types of Flowers Based on Sex
A. Complete (Hermaphrodite) Flowers
Have both stamens and pistil in the same flower
Examples: trillium, raspberry
B. Monoecious Plants
Separate male and female flowers
On the same plant
Examples:
Corn:
Tassel = male flowers
Ear = female flowers
Hazelnut (catkins are male)
C. Dioecious Plants
Male and female flowers on different plants
Examples:
Cannabis
Asparagus
This is like animals.
8. Selfing vs Outcrossing
Selfing
Fertilization using pollen from the same genetic individual.
Leads to:
Inbreeding depression (reduced fitness)
Weak offspring
Outcrossing
Fertilization involving genetically distinct individuals.
Leads to:
Genetic diversity
Vigorous plants (hybrid vigor)
Plants prefer outcrossing but keep selfing as backup.
9. Why Hermaphrodite is Most Common
Due to plant immobility, reliance on external vectors (pollinators) is risky.
If no pollinator is present:
A Hermaphrodite flower can successfully self-fertilize.
This strategy ensures the production of seeds rather than zero reproductive output.
This is often summarized as: Better poor offspring (due to inbreeding depression) than no offspring.
10. How Plants Avoid Selfing (Strategies for Outcrossing)
Three primary mechanisms reduce the risk of selfing in hermaphrodite flowers:
Temporal separation of sexual function (Dichogamy)
Spatial separation of sexual organs (Herkogamy or Heterostyly)
Self-incompatibility (Molecular rejection system; mentioned but for later lecture)
11. Temporal Separation (Dichogamy: different timing of male/female function)
Protogyny (female first)
Phase 1:
Stigma is receptive.
Anthers are closed (pollen not released).
Phase 2:
Stigma loses receptivity and shuts down.
Anthers open and release pollen.
Protandry (male first)
Phase 1:
Anthers release pollen.
Stigma is closed or non-receptive.
Phase 2:
Stigma opens and becomes receptive.
Anthers are empty.
Potential Issue: If a plant produces many flowers, different flowers may be in different phases (e.g., in avocado), potentially leading to cross-flower selfing (geitonogamy).
12. Spatial Separation (Herkogamy)
This involves creating physical distance between the anthers and the stigma within the same flower, reducing the likelihood that pollen will fall or be transferred directly to its own stigma.
Two common configurations:
Approach herkogamy: Long style
Reverse herkogamy: Short style
This physical distance greatly reduces selfing.
13. Special Case: Primrose (Heterostyly)
Heterostyly is a sophisticated form of spatial separation where a species produces two distinct flower morphs that differ in the relative lengths of their styles and stamens. This dimorphism ensures mandatory outcrossing between morphs.
The two morphs found in Primrose species are:
Pin Flowers: Characterized by a long style (stigma high) and short stamens (anthers low).
Thrum Flowers: Characterized by a short style (stigma low) and long stamens (anthers high).
Crucial rules governing heterostyly:
A single plant produces only one morph (either Pin or Thrum).
Pollen from the high anthers of a Thrum flower attaches to a specific, high point on a pollinator's body. This exact position matches the high stigma of a Pin flower.
Similarly, pollen from the low anthers of a Pin flower matches the low stigma of a Thrum flower.
Result: This 'geometrically correct' deposition system minimizes self-pollination (selfing within the same morph is physically unlikely) while highly favoring efficient cross-pollination (outcrossing) between opposite morphs.
14. Overview of Reproductive Systems
The reproductive systems discussed (Hermaphrodite, Monoecious, Dioecious) define the baseline possibility for selfing or outcrossing. While Hermaphroditism is common, mechanisms like dichogamy (temporal separation) and heterostyly (spatial separation) have evolved specifically to circumvent the disadvantages of self-fertilization inherent in having both sexes in one flower or on one plant.
15. Fruit Structure Examples
Cherry / Peach / Olive
1 carpel
1 ovule
1 seed (pit)
Blackberry / Raspberry
Many free carpels
Each becomes a small fruit unit (an aggregate of drupelets)
Cucumber / Zucchini
3 fused carpels
Many ovules
16. Key Takeaway Concepts You Must Know
You must be able to explain:
Free vs fused sepals/petals (Calyx and Corolla structure)
Stamen structure, including the 4 microsporangia and the process of dehiscence
Stigma and style adaptations, including the specialized secretory transmitting tissue
The distinction and hierarchy of Ovary vs ovule vs egg cell
The Carpel vs pistil rule (especially regarding fused carpels creating a single pistil)
Differences in fruit origins: cherry (1 carpel), blackberry (many free carpels), cucumber (3 fused carpels)
Sexual classification: Hermaphrodite (Complete) vs monoecious vs dioecious plants
Why selfing leads to inbreeding depression but is maintained as a reproductive safeguard
Mechanisms of selfing avoidance: Temporal separation (dichogamy: protogyny/protandry) and Spatial separation (herkogamy)
The functioning of Primrose heterostyly as a specialized mechanism for promoting mandatory cross-pollination (outcrossing).