Reproduction in Angiospermophytes Topic 9.3

Assessment Statements 9.3.1 Draw and label a diagram showing the structure of a dicotyledonous animal-pollinated flower. 9.3.2 Distinguish between pollination, fertilization and seed dispersal. 9.3.3 Draw and label a diagram showing the external and internal structure of a named dicotyledonous seed. 9.3.4 Explain the conditions needed for the germination of a typical seed. 9.3.5 Outline the metabolic processes during germination of a starchy seed. 9.3.6 Explain how flowering is controlled in long-day and short-day plants, including the role of phytochrome.

Flowers Reproductive structures of angiospermophytes Dependent upon animals for pollination Can grow as large as 3 feet in diameter and weigh as much as 14.5 lbs. Rafflesia arnoldii When mature, this flower smells like rotting meat thus attracting flies that transfer pollen from the male reproductive structures to the female structures

Flower structure and function Flower part Function Sepals Protect the developing flower while in the bud Petals Often are colorful to attract pollinators Anther Part of stamen which produces the male sex cells, pollen Filament Stalk of stamen that holds up the anther Stigma Sticky top of carpel on which pollen lands Style Structure of the carpel that supports the stigma Ovary Base of carpel in which the female sex cells develop

Carpel (entire female part) Stamen (entire male part) Complete – contain sepals, petals, stamen, and carpal Incomplete – lack at least one part Staminate – have only stamens Carpellate – have only carpels

Alternation of generations All plants show two different generations in their life cycle: Gametophyte generation which is haploid Sporophyte generation which is diploid Gametophyte generation produces plant gametes by mitosis Sporophyte generation produces spores by meiosis For example, a cherry tree is in the sporophyte form (it grew from a zygote and produces new cells by mitosis). When the cherry tree produces flowers, haploid spores are formed and develop into haploid bodies referred to as gametophytes. Sperm form within the male gametophytes, and eggs form within the female gametophytes.

Pollination Process by which pollen (containing male sex cells) is placed on a female stigma First step towards fertilization and the production of seeds Common vectors: wind, insects, birds, water, and other animals Means of attraction Red flowers are conspicuous to birds Yellow and orange flowers are noticed by bees Heavily scented flowers are easily found by nocturnal animals Plants that rely on wind have inconspicuous, odorless flowers

Types of pollination Self-pollination Cross-pollination Pollen from the anther of the same plant falls onto its own stigma Form of inbreeding and results in less genetic variation within a species Pollen is carried from the anther of one plant to the stigma of a different plant Increases variation and may result in offspring with better fitness Problem: distance

Fertilization Pollen grain adheres to the stigma, which is covered by a sticky, sugary substance Pollen germinates to produce a pollen tube Pollen tube grows down the style of the carpel Within the growing pollen tube is the nucleus that will produce the sperm Pollen tube completes its growth by entering an opening at the bottom of the ovary Sperm moves from the tube to combine with the egg of the ovule to form a zygote Zygote develops with the surrounding tissue into the seed As the seed is developing, the ovary around the ovule matures into a fruit The fruit encloses and helps to protect the seed

The seed (means by which an embryo can be dispersed to distant locations) Seed part Function Testa Tough, protective outer coat Cotyledons Seed leaves that function as nutrient storage structures Micropyle Scar of the opening where the pollen tube entered the ovule Embryo root (radicle) and embryo shoot (epicotyl) Become the new plant when germination occurs Plumule will become first leaves. Hilum is where the seed was attached to the ovary. Endosperm provides nutrition for growing embryo.

Maturation Dehydration until water content of the seed is about 10% - 15% of its weight Seed enters dormancy (low metabolism, not growth or development) Adaptation to overcome harsh environmental conditions Conditions needed for germination: Water Oxygen for aerobic respiration Appropriate temperature for enzyme action Other (testa disrupted, fire exposure) Most will not become a functional plant so plants produce large numbers of seeds

Seed metabolism during germination Uptake of water Gibberellin is released Gibberellin (growth hormone) triggers the production of the enzyme amylase Amylase causes the hydrolysis of starch into maltose. The starch is present in the seed’s endosperm Maltose is further hydrolyzed into glucose that can be used for cellular respiration or may be converted into cellulose by condensation reactions. Cellulose is then used to produce the cell walls of new cells being produced

Control of flowering in angiosperms Photoperiodism – plant’s response to light involving the relative lengths of day and night (a very important factor in the control of flowering) A plant must flower when pollinators are available and when necessary resources are plentiful. Plant-type Flowering and light Examples Long-day plants Bloom when days are longest and nights shortest (midsummer) Radishes, spinach, lettuce Short-day plants Bloom in spring, late summer, and autumn when days are shorter Poinsettias, chrysanthemums, asters Day-neutral plants Flower without regard to day length Roses, dandelions, tomatoes

Phytochrome Phytochrome is a photoreceptor, a pigment that plants use to detect light. There are two forms: Inactive (Pr) and active (Pfr) When red light (wavelength of 660 nm) is present in available light, the inactive form Pr is converted into the active form Pfr which has the ability to absorb far-red light (wavelength of 730 nm). This Pfr is rapidly converted back to the inactive form in daylight. However, in darkness, the conversion is very slow. It is thought that this slow conversion of Pfr back to Pr allows the plant to time the dark period.

In long-day plants, the remaining Pfr at the end of a short night stimulates the plant to flower. In this case, Pfr acts as a promoter In short-day plants Pfr appears to act as an inhibitor of flowering. For these short-day plants, enough Pfr has been converted to Pr to allow flowering to occur. Even though the names refer to day length, it is actually the length of night that controls the flowering process.