Flowers, Fruits, and Seeds
What is a flower?
A reproductive shoot that performs the function of pollination, which is when pollen from an anther lands on the stigma of a pistil
The flower is a determinate shoot that has modified leaves for reproduction
Flowers may have both fertile portions (i.e. stamens, carpals) and sterile parts (i.e. sepals, petals).
Flowering plants arose during the Early Cretaceous, at least 130 million years ago, although there have been other plants during evolutionary history that have had flower-like structures and/or used pollinators to facilitate pollination.
What are the functions of flowers?
Flowers of angiosperms perform two functions: pollination and fertilization
Pollination
pollen is transferred from anther (of the stamen) to stigma (of the carpal)
Cross-pollination: pollen comes from a genetically-different plant than the stigma on which it lands
Self-pollination: pollen comes from the same genetic individual as the stigma.
Fertilization
Pollen tube (=male gametophyte) grows through the stigma and style to an ovule
The sperm nuclei fuse with egg in the ovule
After fertilization ovules are called seeds, and the ovary is called a fruit
Above: Pollen grain of lily, with pollen tube (male gametophyte) emerging.
Complete and Perfect Flowers
Complete flowers: A flower that has all four parts (i.e. sepals, petals, stamens, pistil)
Incomplete flowers: A flower that is missing one or more of the four parts
Perfect flowers: A flower that has both sexes (i.e. stamens and pistil)
Imperfect flowers: A flower that is either just male (i.e. stamens) or female (i.e. pistil)
Superior and Inferior Ovaries
Hypogynous: petals attached below ovary; ovary sits on top of receptacle (it is superior to the receptacle)
Perigynous: petals attached above ovary to a cup (called the hypanthium); ovary sits on top of receptacle (it is superior to the receptacle)
Epigynous: petals attached above ovary; ovary embedded in receptacle/hypanthium complex (it is inferior to the receptacle)
What is a fruit?
A fruit is the fertilized ovary of an angiosperm, which performs the function of protecting and dispersing seeds.
Technically, the fruit is the defining feature of the angiosperms, not the flower.
Not all fruits are edible.
Do not confuse a botanical fruit with a culinary definition.
Above: a Venn diagram to explain the similarities and differences between fruits and vegetables
What are the functions of fruits?
Fruits perform two functions: protection and dispersal
Protection
The ripening ovary protects and nourishes the seeds inside
Dispersal
At maturity, the fruit aids in the dispersal of the seeds
How are fruits structured?
Not all fruits have the below layers, but these terms can help differentiate types
Exocarp: the outermost layer of fruit (e.g. rind)
Mesocarp: the middle layer (e.g. pulpy)
Endocarp: the inner layer (e.g. pit)
Pericarp: layers cannot be distinguished in fruit
What are the fruit types?
There are various types that can be found on the fruit type page
What are the growth forms of plants?
Annuals
Plants that complete their life cycle* in one season; adult plants die after 1st year
Biennials
Plants that complete their life cycle* in two seasons; adult plants die after 2nd year
Perennials
Plants that complete their life cycle* each season; adult plants continue each year
Monocarpic plants
Plants that complete life cycle* only once in a lifetime; adult plants continue each year; dies after flowering
Seed-bearing vs. Spore-bearing plants
For more information, see the "diplo-haplontic lifecycle"
Contrary to belief, all plants produce spores (e.g. pollen), and the distinction between spore-bearing and seed-bearing plants is more nuanced.
The main distinction relates to the gamete-producing part of the life cycle. Inside all spores are tiny plants, called the gametophytes ("-phyte" means plant), although they may not appear very plant-like.
Spore-bearing plants generally have gametophytes that are free-living in the environment. They tend to be green/photosynthetic and small. Think of moss... the green leafy moss is the gametophyte
Seed-bearing plants also have gametophytes, but they are not free-living. They remain contained in the spore or protected from the environment, and are not photosynthetic. See below...
Spore-bearing plants
Produce spores through meiosis in a spore case (sporangium).
Each spore germinates on soil to produce a free-living gamete-producing phase (=gametophyte).
Seed-bearing plants
Produce micro-spores, called pollen, through meiosis which germinate on the ovule (gymnosperms) or the stigma (angiosperms) to produce a male gametophyte, called the pollen tube.
Produce mega-spores through meiosis which contains the female gametophyte inside an ovule. The ovule becomes a seed after fertilization.
Laboratory
Capsella embryo
LABEL: Cotyledons, apical meristem, radicle, hypocotyl
Monocot Flower Bud (x.s.)
LABEL: Sepals, petals (how many?), stamens (anther, filament, pollen chamber, pollen), pistil (ovary wall, ovules, locule)
Dicot Flower Bud (x.s.)
LABEL: Sepals, petals (how many?), stamens (anther, filament, pollen chamber, pollen), pistil (ovary wall, ovules, locule)
Questions for Thought
What is a flower in an evolutionarily sense, and what is its function when open?
What is the specific function of each of the flower parts (i.e. sepals, petals, stamens, carpals)?
How is a flower a specialized shoot system?
What is an inflorescence?
What is a perfect flower? What is a complete flower?
How does an imperfect flower prevent inbreeding of a species?
How are the terms perigynous and pome interconnected?
When is an inflorescence the same as a flower?
Is a catkin perfect and complete? Why or why not?
In a composite inflorescence, is a disk flower considered complete? Why or why not?
How does the structure of cymes, corymbs, and composites aid in pollination?
What is the technical difference between a gymnosperm and angiosperm?
For example why is the berry-like structure of a yew (Taxus) still considered a gymnosperm, and the cone-like structure of an alder (Alnus) still an angiosperm.
What is the difference between a seed and an ovule?
What is the function of all fruits?
What is the difference between a simple, aggregate, and multiple fruit?
What does a pulpy or fleshy endocarp usually mean for seed dispersal?
Would you expect wind dispersal to be common in a tropical rain forest? Why or why not?
What are the differences in survival strategies between an annual plant and a perennial plant?
How is a monocarpic life cycle advantageous in extreme environments?
Additional Resources
The Beauty of Pollination (YouTube 7May2011)
Solving Crimes With Pollen, One Grain Of Evidence At A Time (NPR April 25, 2015)
The Queen of Trees: a fascinating documentary about an African fig tree (YouTube)
Frogs that pollinate flowers (28Apr2023 NY Times)
