14 August 2006

What Are Flowers For?

Picture of faba bean seeds from site of the Department of Primary Industries of the State of Victoria at http://www.dpi.vic.gov.au/dpi/nrenfa.nsf/LinkView/447D803B6115D09ACA256FFF0082AAE53E07C6C441BF771A4A2567D80005AA20

Flowers, sex and seeds

Flowers are the specialized plant structures which produce pollen and where seeds develop within an enclosing fruit. Each seed (like the faba bean seeds at the right) contains a baby plant.

A baby plant (plant embryo), and its surrounding seed, cannot develop unless pollen is transferred from the pollen-producing parts of the flower to the parts which contain ovules, which can become seeds.

So flowers serve two basic purposes:
  • they package genetic material (into pollen and ovules) and help move it around so it can combine to produce the seeds for the next generation, and

  • they enclose those seeds in a fruit to help them successfully grow into a new plant.

Picture of peanuts from Purdue University site http://www.hort.purdue.edu/ext/senior/fruits/peanut2.htmIf you look closely at this picture of some peanuts, you can see a baby plant where one of the peanut seeds has split in two. Each seed was enclosed in a "seed coat" (the red, papery covering) and several seeds were in each fruit (the peanut shell with the seeds inside -- not shown). (Better yet -- get some peanuts and look at them carefully. This is called "observation" -- a big part of science.)

Not all plants have flowers

Some plants don't make seeds at all (like ferns or mosses). And others (gymnosperms like pines, ginkgos and cycads) make seeds without flowers. (The seeds of gymnosperms aren't enclosed in a fruit that develops from part of a flower. They are just borne externally, although a cone or fleshy outgrowth may surround them as they mature.)

Flowers facilitate plant sex

They typically include structures which produce pollen and other structures where seeds can develop if pollen reaches them. Sexual reproduction means the parent organisms produce “gametes”, which carry a sample of the parent's genetic information (just half of it, but one of every chromosome). These gametes must unite to reconstitute the complete genetic complement that the next generation will need—two of each chromosome. In people the gametes are sperm and the egg. In seed plants they are pollen and ovules.

Usually a seed can develop from union of pollen and an ovule on the same plant. But many flowering plants promote broader mixing of genetic material. They have have forms which encourage the transfer of pollen from one flower to another, and thus often from one plant to another. This transfer of pollen can be done by wind, or by birds, insects or other animals which visit the flowers. Many flowers have evolved specific forms, colors, or other features to attract such “pollinators”.

After pollination (the transfer of pollen to the parts of the flower where the ovule is waiting) and fertilization (the joining of the genetic material from the two gametes), a seed can develop. In flowering plants, seeds are enclosed in a structure called a fruit. A fruit can contain just one seed (like an olive) or many (like a tomato). A fruit can be dry and hard, like a grain of rice, or fleshy like an apple.

Dandelion fruits
Many fruits have specialized structures to help carry the seeds away from their parent plants, like a dandelion (which floats on the wind), burdock (which has prickles to catch on fur of passing animals), Impatiens (“touch-me-not”, where the fruit explodes and scatters the seeds), coconuts (which can float), or the cherry (which has an edible portion surrounding a tough digestion-resistant seed).

So flowers make fruits, and a fruit has three parts:

  • the embryo (baby plant)

  • the seed that contains the embryo

  • the fruit that contains the seed (or seeds). The fruit develops from the structure that contained the ovules before fertilization.

More about plant sex

Flowers are all about sex. Sexual reproduction produces offspring which are not genetically identical to their parents. In fact, each offspring can contain a novel combination of genes never seen before. This helps plants deal with changing environments and other challenges (new competitors, new predators). The plant gets the most novel new combinations if pollen is transferred from one parent to another. Plant sexuality is very diverse, with many complicated sexual mechanisms even just within the flowering plants. These various mechanisms include differences in which flowers produce pollen and which produce ovules, whether the pollen-producing flowers and the ovule-producing flowers are even on the same plant, where within the flowers pollen is released and where it can be received, when the pollen is released and when the ovules are receptive, how the pollen is transferred, and chemical signals which determine which pollen will be allowed to fertilize an ovule. The incredible diversity of the flowering plants (90% of all plants living today, comprising hundreds of thousands of species) is all about trying new ways to have sex.

Although many plants don't use pollinators such as insects (wind pollination is common), the fossil record suggests that flowers and insects evolved together, and that the diversity of each stimulated diversity in the other.

Flowers work

Flowering plants provide the basis of nearly all human nutrition, except for the wild-caught fish we eat. (A few calories also come from algae, pine nuts, fern fiddleheads and the like.) Without flowering plants civilization would certainly be impossible with today's technology. (Could a civilization develop that depended entirely on fish for nutrition?) Flowers, seeds and fruits have, ultimately, made the internet, and every other aspect of civilization, possible.

To review:

Flowers have pollen. If pollen is transferred, flowers grow seeds, and turn into fruits.

Homework

So next time you are looking at a flower, try to see where the pollen is and where it might go to make a seed. How does pollination happen?

Next time you are looking at a fruit, consider how it formed and how it helps its seeds get around. Where are the seeds and how are they dispersed?

Next time you consider a seed, think about the baby plant inside, and how it might grow into a mature plant with flowers of its own.



Here are some links for further information:

Wikipedia: flowers, flowering plants, seeds
Parts of a flower (floral structure)
Good pdf document about floral structure
Good summary page with images of various flowers
Diversity of flowering plants
The Seed Biology Place

The photo of dandelion fruits is by Andreas Trepte licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license. Rights information here.


David Wheat's Science In Action site has articles about science and math in the real world, weird science, science news, unexpected connections, and other cool science stuff. There is an index of the articles by topic here.

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1 comment:

still learning said...

Thank you - it was very helpful.