The Flower, Pollination, the Seed, and Photoperiodism

Sexual Reproduction in Angiosperms

Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely.

There must also be a mechanism to disperse their offspring far enough away from the parent so that they do not have to compete with the parent for light, water, and soil minerals.

The functions of the flower solve both of these problems.

Flower Power!

Life of the Flower

PollinationThe act of pollen (male gamete) being

transferred from anther to stigmaUsually accomplished by wind or

animals

The Flower The flower consists of 4 concentric whorls of

tissue

1. a whorl of sepals (collectively called the calyx)

2. a whorl of petals (collectively called the corolla)

3. stamens in which the microsporangia (sperm) form

4. carpels in which the megasporangia (eggs) form.

Anatomy of the FlowerNon-reproductive Structures

Sepals: protectionPetals: attracts

pollinatorsPedicel: connects

flower to stem

Anatomy of the FlowerMale structuresAnther: produces pollen Filament: supports and

lifts antherStamen = anther +

filament

Female structures (collectively known as carpels)Stigma: attachment site

for pollenStyle: connects stigma

to ovaries; has pollen tube

Ovary: location of ovules

SEM of various pollen grains

Sperm vs. EggMale gameteHaploidProduced in anthersCalled

microsporangium or pollen

Each pollen grain has two cellsGerm cellTube cell

Female gameteHaploidProduced in ovaryCalled

megasporangium or ovule

Each ovule consists of 8 nuclei

Fertilization: The fusion of a male gamete (pollen) with female gamete (ovule) to produce a zygote

Life Cycle of Angiosperms

Dicot Seed Anatomy5-15% waterCan remain

dormant and viable up to 300 yrs (species dependent)

Seed Coat: testa

Embryonic Leaf:plumule

Embryonic Root:radicle

Conditions of GerminationFor successful germination seeds require a

combination of …oxygen for aerobic respiration water to metabolically activate the cells temperature for optimal function of

enzymes

Each seed has its own particular combination of the above three factors.

In addition particular species need other more specialized conditions such as:

fire freezing passing through digestive system of a seed dispersing

animal washing to remove inhibitors (beans) erosion of the seed coat (Poppy)

The particular conditions required by a seed allows it to match germination to favourable conditions

Seed Germination1. Rehydration of cells stimulates metabolism

(enzymatic pathways startup)

2. Gibberellin (hormone) produced in cotyledon

3. Gibberellin stimulates amylase production4. Starch is digested in maltose5. Maltose is transported to radicle and

plumule6. Maltose converted to glucose, used for

cellular respiration or cellulose production

Photoperiodism Many plants have the ability to measure the length of the sun cycle.

This allows them to schedule their flowering period for a specific time of year

Experiments have proven that plants measure night length, not day

PhotoperiodismLong Day Plants

(LDP)Plants will only

flower after a minimum amount of daylight is achieved.

Typical of plants living in higher altitudes

Usually during spring or summer

Short Day Plants (SDP)Plants that only

flower when the day is shorter than a maximum time period

Usually during fall or winter

Photoperiodism also explains why some plant species can be grown only in a certain latitude.

Spinach, a long-day plant, cannot flower in the tropics because the days never get long enough (14 hours)

Ragweed, a short-day plant, fails to thrive in northern Maine because by the time the days become short enough to initiate flowering, a killing frost in apt to occur before reproduction and the formation of seeds is completed.

Phytochrome System: The receptor of photoperiod is located within

the leaf.The cellular location of the receptor is

unclear.The chemical nature of the receptor is a the

molecule phytochrome. Phytochrome can be converted from one form

to another by different types of light.

Flowering in SDPShort day plants flower when the night period is long. In day light or red light, Phytochrome Red (Pr) is

converted to Phytochrome Far Red (Pfr). The conversion actually only requires a brief exposure to

white or red light.

In the dark, Pfr is slowly converted back to Pr. A long night means that there is a long time for the

conversion.

Under short day conditions (long night) at the end of the night period the concentration of Pfr is low.

In SDP, low Pfr concentration is the trigger for flowering.

Flowering in LDP:Long day plants flower when the night period is short.

In day light (white or red) the Pr is converted to Pfr.

During periods when the day light period is long but critically the dark period is short, Pfr does not have long to breakdown in the dark. Consequently there remains a higher concentration

of Pfr.

In LDP, high Pfr concentration is the trigger to flowering.