Plant Reproduction AP Biology Unit 5

Alternation of Generations All land plants can exist in two different multicellular forms Sporophyte (2n) Gametophyte (n) In a life cycle, plant generations alternate between sporophyte and gametophyte Each form gives rise to the other form

Sporophyte vs. Gametophyte Sporophyte (2n) Diploid, multicellular form of a plant Forms haploid spores through meiosis Gametophyte (n) Haploid, multicellular form of a plant Forms gametes through mitosis

Alternation of Generation

How is this different from other organisms? In other organisms (like humans) the haploids are unicellular (sperm and egg) In plants both forms (sporophyte and gametophyte) can be multicellular

Evolution of Plant Reproduction Each type of plant has a dominant form (sporophyte or gametophyte) Dominant form = the “plant” we think of As plants have evolved, there have been changes to relationship between sporophyte and gametophyte

Gametophyte/Sporophyte Relationships 3 different possibilities Larger gametophyte, sporophyte is dependent on it Larger sporophyte, gametophyte is independent Larger sporophyte, gametophyte is dependent

Question… What trend do you see between the evolution of plants (from nonvascular  vascular; nonseed  seed plants) and the alternation of generations? As plants have evolved, the gametophyte has become more reduced, sporophyte has become more dominant

Reproductive Organs Archegonium Antheridium Female sex organ  produces egg Antheridium Male sex organ  produces sperm

Vascular Plants: Homosporous vs. Heterosporous Vascular plants have sporophylls modified leaves that produce sporangia (which become spores) Homosporous plants Most ferns Heterosporous plants All seed plants (angiosperms and gymnosperms) and some seedless plants

Water vs. Wind Mosses and Ferns require water to help the sperm reach the egg. Seed plants no longer require water to reproduce Pollen (male gametophyte) can be carried by wind, animals, etc.

Gymnosperm Life Cycle Sporophyte is the dominant form Heterosporous Have “naked” seeds – nothing around the seed Cones are either male (contain pollen) or female (contain ovule)

Flower Anatomy The flower is the reproductive organ of an angiosperm Female part = Carpel Consists of the stigma, style, and ovary Male part = Stamen Consists of anther and filament

Angiosperms Dominant form is the sporophyte Heterosporous Male gametophyte = pollen grain Female gametophyte = embryo sac

Female Gametophyte The megaspore will go through MITOSIS to create a series of different cells in the female gametophyte Cells involved in fertilization are Egg (n)– haploid Polar nuclei (2n) – 2 nuclei in one cell

When the pollen grain lands on the stigma, it will extend a pollen tube down the style to get to the egg Specific cells in the pollen tube go through mitosis to form 2 sperm Sperm will enter

Double Fertilization Angiosperm fertilization actually consists of 2 separate fertilization events = double fertilization Both sperm in the pollen tube are used One sperm fertilizes the egg  diploid zygote (2n) forms The other sperm fertilizes the polar nuclei (2n)  forms triploid endosperm (3n)

Double Fertilization Double fertilization is unique to angiosperms Through this process, resources are used most efficiently since endosperm can only develop in a fertilized ovule. Endosperm = starchy material that serves as nutrition for the growing embryo

Other Post-Fertilization events Embryo develops (diploid) Seed coat forms Ovary develops into fruit around the seed

Germination and Seedling Formation Seeds are dormant until they begin germination When conditions are right, seeds will begin to germinate to form a seedling. Germination starts with imbibition—when water enters the seed and triggers enzymes that are needed for growth.

Seed Benefits What are the evolutionary benefits of seeds? Dispersal- embryo can travel far from parent Protection- seed coat protects embryo Dormancy- can wait until conditions are favorable to germinate Storage- food for the embryo can be stored inside seed (ex. endosperm)

Monocots vs. Eudicots Most angiosperms can be classified as either Monocots or Eudicots