Plant Reproduction

Flower Production

Four genetically regulated pathways to flowering have been identified

The $light$ -dependent pathway
The $temperature$ -dependent pathway
The $hormone-dependent$ pathway ((gibberellin)
The $autonomous$ pathway (epigenetic and post-transcriptional)

Plants can rely primarily on one pathway, but all four pathways can be present

Flower Structure

Floral organs are thought to have evolved from leaves

A complete flower has four whorls

$Calyx, corolla, androecium$ , and $gynoecium$ .

An $incomplete flower$ lacks one or more of these whorls.

Trends in Floral Specialization

2 major trends floral specialization:

Floral parts have grouped together
Floral parts lost or reduced

Modifications often relate to pollination mechanisms

Trends in floral symmetry:

Primitive flowers are radially symmetrical

Advanced flowers are bilaterally symmetrical

Trends in Pollination

Process by which pollen is placed on the stigma

$Self-pollination$

Pollen from a flower’s anther pollinates stigma of the same flower.

$Cross-pollination$

Pollen from anther of one flower pollinates another flower’s stigma.
Also termed outcrossing.

Successful Pollination

Successful pollination in many angiosperms depends on regular attraction of $pollinators$
Floral morphology has coevolved with pollinators
Early seed plants wind pollinated
Among insect-pollinated angiosperms, the most numerous groups are those pollinated by bees

Bee Pollination

Bees typically visit yellow or blue flowers
Many have stripes or lines of dots that indicate the location of the $nectaries$ (specialized nectar-producing structures).

Other Pollinators

Flowers that are visited regularly by butterflies often have flat “landing platforms”
Flowers that are visited regularly by moths are often white or pale in color and also tend to be heavily scented, making them easy to locate at night
Flowers that are visited regularly by birds must produce large amounts of nectar
Often have a red color * Conspicuous to birds, but usually inconspicuous to insects.

Wind Pollination

Some angiosperms are wind-pollinated

Characteristic of early seed plants.

Flowers are small, green, and odorless, with reduced or absent corollas

Often grouped and hanging down in tassels

Stamen- and carpel-containing flowers are usually separated between individuals

Strategy that greatly promotes outcrossing.

Seeds

In many angiosperms, development of the embryo is arrested soon after $meristems$ and $cotyledons$ $differentiate$
$Integuments$ develop into a relatively impermeable $seed coat$
Encloses the seed with its dormant embryo and stored food

Initiation of Germination

$Germination$ : the emergence of the $radicle$ (first root) through the seed coat
Germination cannot take place until water and oxygen reach the embryo
$Stratification$ : some seeds require periods of time at low temperatures before germination

Food Storage in the Seed

Germination and early seeding growth require the metabolism of energy reserves (starch)
$Scutellum$ : in kernels of cereal grains (monocots), the single cotyledon is modified into this structure, which transfers nutrients from the endosperm to the embryo

Hormonal Effects

In response to the absorption of water by a seed, the embryo produces $gibberellins$ .
This signals the outer layer of the endosperm to produce $amylase$ * Amylase digests starch.
Levels of $abscisic acid$ (another hormone that produces seed dormancy and starch breakdown ) may be reduced when a seed absorbs water

Asexual Reproduction

Produces genetically identical individuals because only mitosis occurs
More common in harsh environments.
$Apomixis$ – asexual development of a diploid embryo in the ovule.
Other interesting (and useful) variations to the life cycle:
$Parthenocarpy$ – inducing fruit formation w/o fertilization w/in the ovule (can be induced in some hormones applied artificially.
$Stenospermocarpy$ – fertilization occurs but development is aborted (for example, some cultivars of seedless grapes).

Vegetative Reproduction

New plant individuals are cloned from parts of adults

Comes in many and varied forms

Runners or stolons.
Rhizomes.
Suckers.
Adventitious plantlets.

Plant Life Spans

Once established, plants live for variable periods of time, depending on the species
$Woody plants$ , which have extensive secondary growth, typically live longer than herbaceous plants, which don’t * Bristlecone pine, for example, can live upward of 4,000 years.
Depending on the length of their life cycles, herbaceous plants may be annual, biennial, or perennial

Annual, Biennial, or Perennial

$Annual$ plants grow, flower, and form fruits and seeds within one growing season

They then die when the process is complete.

$Biennial$ plants have life cycles that take two years to complete

Store energy in year one, flower in year two.

$Perennial$ plants continue to grow year after year

They may be herbaceous or woody.