Plant Reproduction & Development Learning Objectives

Microsporangia

Found in the anther; contain microsporocytes (2n) that undergo meiosis → microspores (n)

Megasporangia

Found within ovules inside the ovary; contain megasporocytes (2n) that undergo meiosis → megaspore (n).

Microsporocyte (2n)

Produces 4 haploid microspores → develop into pollen grains (male gametophytes)

Megasporocyte (2n)

Produces 4 haploid megaspores, but only one survives → develops into female gametophyte (embryo sac)

Microspore

pollen grain (male gametophyte) with two cells

Tube Cell

forms the pollen tube

Generative Cell

divides into 2 sperm cells

Megaspore

embryo sac

Embryo sac has

1 egg cell, 2 synergids, 2 polar nuclei, 3 antipodal cells

Pollen

Develops in anther; lands on stigma → germinates → pollen tube grows down style to ovule

Ovule

Located in ovary; contains egg cell and supporting cells; opening called the micropyle allows pollen tube entry

Double Fertilization Step 1

One sperm fertilizes egg → zygote (2n)

Double Fertilization Step 2

Other sperm fuses with two polar nuclei → endosperm (3n) (nutrient tissue)

cotyledons dicot

store nutrients

radicle dicot

embryonic root

hypocotyl dicot

stem region below cotyledons

epicotyl dicot

stem region above cotyledons

plumule dicot

embryonic shoot

seed coat dicot

protective covering

micropyle dicot

small opening where fertilization occurred

cotyledon monocot

Transfers nutrients from endosperm to embryo

radicle monocot

embryonic root

epicotyl monocot

embryonic shoot

hypocotyl

between radicle and cotyledon

coleoptile monocot

protective sheath covering emerging shoot

coleorhiza monocot

protective sheath around radicle

endosperm monocot

food source for embryo

seed coat monocot

protective layer

reception

A signal molecule (hormone or environmental cue) binds to a receptor protein in the plasma membrane or cytoplasm

transduction

Relay molecules (often involving second messengers like Ca²⁺ or cyclic GMP) amplify the signal

response

Activation of specific genes or enzymes → changes in growth, metabolism, or development (e.g., phototropism, stomatal movement)

Darwin & Darwin

Found that the coleoptile tip detects light and sends a signal to the growth region. When the tip was covered or removed, no bending occurred → tip = light receptor

Boysen-Jensen

Separated coleoptile tip using agar (permeable) or mica (impermeable). Bending occurred when separated by agar, but not mica → tip releases a diffusible chemical

Went

Placed agar block (with chemical) on one side of coleoptile → bending occurred. Identified the diffusible substance as auxin (IAA – indoleacetic acid), which promotes cell elongation on the shaded side

Action Spectrum for Phototropism

Shows that blue light causes the strongest phototropic response.
Indicates involvement of blue-light receptors (phototropins) in detecting directional light