Evolution of the seed

Adaptations of seed plants

• Seeds• Reduced gametophyte stage

– Also seen in seedless vascular– seed plants reduced to microscopic (spores develop in

sporangia of sporophyte)• Heterospory

– Megaspores – make female gametophyte – Microsporangia – male gametophyte

• Ovules = megaspore & integuments (protect) • Pollen = micro (male gametophyte) & sporopollenin

Microsporangium: on anther

Megasporangium

• Ovary produces the ovule• Ovule parts:

– INTEGUMENT– MACROSPORAGIA– FEMALE GAMETOPHYTE (egg)

Seed

• Seed :– Embryo: immature plant– Endosperm:food supply – Seed coat of integument

• Ovule develops into seed• Cotyledons: 1 or 2 seed leaves

• Seed parts• Embryo• Endosperm• Seed coat

Seed

• Double Fertilization– The union of two

sperm cells with different nuclei of the embryo sac.

• Endosperm– Food storing tissue of

the seed.

http://www.learner.org/channel/courses/essential/life/images/show4.open_seed.jpg

Seed to Seedling

• Imbibition is that state when the seed wakes up from dormancy and starts the intake of water.

• It causes the seed to expand and ruptures, releasing the shoot, the cotyledons and the stalk.

• It is the first sign of life after the dormant state.

Seed to Seedlinghttp://students.usm.maine.edu/deidre.rice/_ILLUS_ILT_T630888A.GIF

Spore survival

• Prior to seeds: spore was the only protected stage for plants.– Spore is single cell– Could survive: cold hot dry– Could be dispersed to another environment

Spore Release

Then, seeds

• Seeds are multicellular• Has protective integument coat from ovule• Can be dormant for days months years

Seed Release

Vascular Seed plants

• Gymnosperms: fir, pine• Angiosperm: flowering plants

Pine tree

• The pine tree is the sporophyte (2N)• Its sporangia are located on the pine cone• Heterosporous

– Small pollen cones: carried by wind and critters– Large ovule cones

Each tree has both types of cones

Female - Male

female male

Angiosperm

• Flowering plants: vascular and seeds• Most diverse and widespread plants• Flower: specialized for reproduction

– Carried by wind: like gymnosperm– Carried by insects: more directed pollination

• Fruit: protect seed and aid in dispersal– From ovary. Ovary thickens after pollination.

Flower Structure

• Four main organs: sepals, petals, stamens, and carpels

• Stamens and carpels are reproductive

• Carpel: – Stigma: collects pollen– Style: to ovary with 1 or

more ovules – ovary– Pistil: multiple fused carpels

• Stamen: filament, anther– Anther: stalk where pollen

made

• Sepals and petals: no repro organs

http://images.google.com/imgres?imgurl=http://andromeda.cavehill.uwi.edu/

Gametophyte Development and Pollination

• Sporangia- structure on – anther and – ovules where spores are produced

• Pollen grains are made up of mature male gametophytes that are enclosed by a spore wall – Found in the microsporangia, or pollen sacs

Hormones

• Auxin– Stem elongation, root growth, develop fruit

• Cytokinins: affect, cell division, cell differentiation, and apical dominance.

• Gibberellins – Seed and bud germination

• Abscisic acid – Inhibit root growth, close stomata

• Ethylene – Fruit ripening

Mechanisms that Prevent Self-Fertilization

• Sexual reproduction ensures that there will be genetic diversity among offspring

• “Selfing” refers to self-fertilization in plants• Ensures that seed will develop

Pollination Vs. Fertilization

• Pollination is the transfer of pollen from anther to stigma (of other plant) – Results in formation of a pollen tube – Purpose is to grow down to ovary and release

sperm within embryo sac– This fertilizes the egg – Embryo -> seed -> fruit containing seed– The fruit disperses seeds which germinate and

develop into seedlings

Development

• Microsporocytes form four haploid microspores – These develop into haploid male gametophytes – The microspore goes through mitosis and

cytokinesis – This results in a generative cell and tube cell which

make up the pollen grain

• Megasporocyte grows resulting in four haploid megaspores – May form a multicellular female gametophyte

• Contains 3 cells: 1 egg and 2 synegrids • Synegrids attract and guide pollen tubes to the embryo

sac • there are also two nuclei at the other end of the cell

that are not separate and share cytoplasm embryo sacs• These result in two integuments that form a seed

• Ensuring that the egg and sperm cells come from different parents is a mechanism that inhibits self fertilization

• Dioecious plants cannot self fertilize because they have either staminate or carpellate flowers

Avoid self fertilization

• Flowers with functional stamen and carpels have organs that mature at different rates– An animal pollinator would not transfer pollen

from the anther to a stigma of the same flower

Self Incompatibility

• The ability of a plant to reject its own pollen• Also, in some cases, the pollen of similar

plants • If pollen were to land on the stigma of a

flower on the same plant a biochemical would prevent the pollen from developing and fertilizing an egg

Special Circumstances

• Plant breeders will hybridize different crop varieties to combine the best traits and to get sufficient results

• This can result in inbreeding • May cause plants that are able to self fertilize

to lose that characteristic.