9.3 – Reproduction in Angiospermophytes

Flowers Flowers – reproductive structures of angiospermophytes Flowers evolved from modified leaves and stems Non-Reproductive Floral Structures: Sepals – “leaves,” at base of flower – enclose the flower before it opens Petals – brightly colored structures that aid in attracting birds and insects Both sepals and petals are not directly involved in reproduction 9.3.1

Floral Structure 9.3.1

Reproductive Structures Reproductive Floral Structures: Stamen – male reproductive structure Anther – sac where pollen in produced Filament – stalk that supports anther Carpel (Pistil) – female reproductive structure Stigma – sticky area on top of carpel that receives pollen Style – tube that connects stigma to ovary Ovary – base of carpel that contains ovule and egg sac 9.3.1

Floral Structure 9.3.1

Control of Flowering Photoperiodism – plant response to light involving relative lengths of day and night. Very important factor in flowering – why? 9.3.6

Control of Flowering Long-day plants – bloom when days are longest and nights are shortest (mid-summer) Short-day plants – bloom in spring, late summer, and autumn when days are shorter and nights are longer Day-neutral plants – day-length not important for flowering Day length is not as critical as night length in regulation of flowering. 9.3.6

Control of Flowering Control by light is due to a pigment in plants called phytochrome. Phytochrome – blue-green pigment that controls various growth responses (including flowering) in plants Two forms of phytochrome: Pr – inactive form Pfr – active form 9.3.6

Control of Flowering Phytochrome is converted from inactive to active forms due to different light wavelengths: In red light (660 nm) inactive Pr is rapidly converted to active Pfr. Active Pfr can absorb far-red light (730 nm) In daylight, Pfr is rapidly converted to back to Pr In darkness, Pfr is very slowly converted back to Pr The slow conversion of Pfr to Pr helps plants time the night length. 9.3.6

Control of Flowering 9.3.6

Control of Flowering In long-day plants, the remaining Pfr at the end of the night stimulates flowering Pfr remains due to slow conversion at night In short-day plants, the remaining Pfr at the end of the night inhibits flowering Short-day plants only flower when enough Pfr has been converted to Pr. (this only occurs during long nights) 9.3.6

Control of Flowering 9.3.6

Pollination Process of moving pollen grains from the anther to the sticky stigma on the carpel Plants have evolved numerous adaptations for pollination Wind pollinated plants produce large amounts of pollen = allergies! Why do they do this? Different colors and shapes of flowers allow plants to attract different pollinators 9.3.2

Pollination What we see… What insects see…

Pollination Most plants have evolved adaptations to limit self-pollination – why is this important? Stamens and carpels may mature at different times Floral structure makes self pollination difficult Flowers are self-incompatible – biochemical mechanisms block prevents pollination

Fertilization Process of fusing sperm and egg to produce new embryo After pollination – the pollen grain produces a tube that extends down the style toward the ovary Growth of the tube is directed by a chemical attractant (usually calcium produced by the ovary) Sperm within the pollen grain fertilize the egg and produce the plant embryo Pollination does not always lead to fertilization 9.3.2

Seed Structure Seed coat (testa) – hard shell that encases and protects embryo Radicle (hypocotyl) – embryonic root Plumule (epicotyl) – embryonic shoot Cotyledons – seed leaves Dicots have two seed leaves Monocots have one seed leaf 9.3.3

Seed Structure 9.3.3

Seed Dormancy Evolutionary adaptation that germination (sprouting) will take place when conditions are favorable For example: Desert plants will only germinate after significant rainfall Many forest species require heat from fire to germinate – how is this an advantage? In areas with harsh winters – seed require a long “chilling,” period before germinating – why? 9.3.4

Germination Begins with imbibition – the absorption of water Causes seed coat to rupture and triggers metabolic changes within embryo The embryo begins producing a plant growth hormone – gibberellins Gibberellins stimulate the production of amylase in the seed Amylase begins digesting food stores of the endosperm (starch) into maltose 9.3.5

Germination Maltose is transferred to the growing regions of the embryo – the embryonic root (radicle) and shoot (plumule) Maltose is converted to glucose – where it used in aerobic respiration for energy or it is used to make cellulose and other materials needed for growth Once the seedling breaks ground, photosynthesis can begin and the endosperm food stores are not needed 9.3.5

Germination

Factors Affecting Germination Water – must be available for imbibition Oxygen – required for aerobic respiration during germination Temperature – enzyme action (amylase) can be affected by fluctuations in temperature 9.3.4