Some plants can reproduce vegetatively – or by producing clones. For many other plants with sexual reproduction, survival depends on germination of seeds and viability of young plants. If a seed germinates in the wrong place or at the wrong time – then life’s over for that individual.
Some plants produce thousands of seeds, which increases the chance that some will germinate successfully.
 Viability Time o seeds vary in the length of time that they can survive without sprouting o some maple seeds need to sprout within weeks o seeds of lotus plants have survived for over 2,000 years
 Dormancy o some seeds must remain dormant before germinating o common vegetable seeds usually lack a dormancy period o seeds from nature often have a dormancy period
 Moisture o moisture triggers germination
 Temperature o seeds of many temperate plants need prolonged cold to germinate; this allows
 germination inhibitors in the seed to slowly disappear
Chemical Inhibitors (chemicals in the seed that inhibit germination) o Abscisic acid (ABA)
 common in temperate species
 induces dormancy in embryos
 abundant in late summer/early fall
 during the winter, enzymes slowly degrade the ABA o Phenolic Compounds
 common in desert plants
 water soluble
 allow germination only when water is present; leach out and allow germination

Dormancy is an adaptation that prevents seeds from germinating at the wrong time, such as during the middle of winter during a warm spell or temporary thaw. Dormancy can be maintained by a variety of factors:

 thick seed coats that keep water out the presence of chemical inhibitors (e.g., phenolic compounds, abscisic acid) that need to
 be broken down for germination to occur photosensitive pigments that must be activated by specific amounts and types of wavelengths insufficient development of the embryo (e.g., some orchid seeds are the size of dust and  have no food stored; they rely on associations with fungi in the soil to feed the developing embryo until it is mature enough to penetrate the seed coat)
Some seeds need to have their seed coats disrupted for water to enter the seed and activate the germination process. The abrading or scarification of seed coats can occur by falling from trees or by passing through the gut of vertebrates.
Cotyledon
Epicotyl
Radicle
Seeds have 3 parts
 A seed coat (fruit coat)
 A storage area (e.g., endosperm)
 An embryo o Cotyledon (seed leaf) – 1 or 2 o Epicotyl – becomes the shoot o Radicle or Hypocotyl – becomes the root
Endosperm
Aleurone
Layer
Seed Coat

1.
Imbibition – water penetrates the seed coat, softens the seed tissue inside, and causes the seed to swell.
2.
The water dissolves the plant hormone, gibberellic acid (GA), inside the embryo. GA then moves throughout the rest of the seed tissue.
 other things can activate GA besides water (e.g., light)
3.
GA enters the cells of the aleurone layer and turns on certain genes in the nuclei of the aleurone cells.
4.
Enzyme genes (e.g., amylase genes) are transcribed into mRNA molecules which move from the nucleus to the cytoplasm.
5.
The mRNA hooks up with ribosomes to make amylase (an enzyme) – translation.
 proteases in the aleurone cells convert storage proteins into amino acids used to make the amylase molecules
6.
Amylase moves from the cytoplasm of the aleurone cells into the endosperm, where it catalyzes the conversion of starch into simple sugars.
7.
The sugars are transported to the embryo and used as fuel for growth.
8.
Once the radicle (hypocotyls) emerges from the soil, it straitens out in response to light absorbed by pigment in the cells.
 red light absorbed by phytochrome
 blue light absorbed by cryptochrome)
The smallest seeds are produced by epiphytic orchids in tropical rain forests. They’re the size of fungal spores (see figure). Small seeds have limited food reserves, so these species must rely on other means to feed the embryos. Orchids establish a symbiotic relationship with mycorrhizal fungi that provide food for the germinating orchid.
Donald J. Leopold
The largest seeds come from the coco-de-mer palm ( Lodoicea maldivica ) found in the Seychelles
Archipelago in the Indian Ocean. A single seed may be 12 inches long, 3 ft in circumference, and weigh 40 pounds.
Some plants produce few seeds, whereas others produce massive amounts. Orchids can produce millions of seeds per plant. Common mullein can produce over 200,000 seeds that can survive for nearly 40 years in the soil.