Pollination, Fertilisation and Variation

Short version:

Flowers allow for variation in plant offspring. This is to increase the chance of the DNA of the plant surviving in a variety of environments and against diseases. Flowers often have mechanisms that encourage movement of its pollen to flowers of other varieties. When the pollen lands, the egg cell in the ovary of the flower is fertilised. The ovary then will develop into tasty fruit so that an animal or bird can then disperse the seed. Without fertilisation the fruit will not develop. Therefore pollinators, such are bees, need to be cared for and not poisoned with pesticides. This cross pollination is the reason why seeds do not always grow plants that give fruit that is the same as the fruit that the seeds came from.  

Plant Reproduction

There are 2 types of reproduction in plants asexual and sexual.

Asexual

Asexual includes tubers, runners, when we take cuttings – basically anything that doesn’t involve a flower. A plant that has reproduced in this way (or plants that we have reproduced in this way) are clones of the parent plant. This is generally considered a good thing in growing food as we want to create new plants from plants with desirable characteristics. This could be a bad thing in terms of diseases. A disease that would kill the parent plant would kill all the clone offspring plants.

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Sexual
The flower allows for sexual reproduction.

For growth and repair in organisms, cells divide into 2 exact copies with exactly the same DNA in each copy. This is called mitosis and happens throughout living things.

Another type of cell division, called meiosis, only happens in the sexual organs. In humans this will be in testicles and in ovaries. In plants this will only be in flowers – in anther sacs of the stamen and in the ovule of the carpel. In meiosis the resulting cells only have half the amount of DNA as the parent cell. One of these cells (with half the DNA) from each parent join to make a new individual with a complete set of DNA. This is how variation occurs.

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Variation

The entirety of a cell’s DNA (all its genetic material) is split between the chromosomes which occur in pairs. Humans have 46 chromosomes, so that’s 23 pairs. The number of chromosomes depends on the species. In Wikipedia I’ve read that pineapples have 50 chromosomes, artichokes have 34, tomatoes have 24 and spinach only has 12. 12 can still provide a lot of variation.

Let’s look at just 4 chromosomes so that’s 2 pairs in each parent. In this example each parent has 2 red and 2 blue chromosomes. They are called A, B, C and D just to show that they are different.

That’s just with 4 chromosomes! With 23 pairs in humans, there would be 23²= 529 combinations for half of the DNA and then 529×529=279,841 possible offspring. In addition to all these possibilities, to add to the mayhem, chromosomes in meiosis may swap bits of their DNA. When the red A and B pair split A might have a bit of its DNA swapped with B. This then adds to the variation.

Pollination and the importance of bees

When the pollen is ready, and the anther sacs are ripe they burst open. The pollen must then be transferred from the male part to the female part. This can be done by the wind (in the case of grass and some trees) or by insects. When the pollen lands on the sticky stigma it sends out a pollen tube, down the style, into the ovary, so that the sperm cell (containing half the DNA) can travel to the egg cell (containing the other half of the DNA) to fertilise it (creating a new individual). Once the egg cell is fertilised, the ovule develops into a seed and the ovary develops into a fruit. Fruit is a method of seed dispersal so any vegetable that has seeds in it is technically a fruit. The fruit is tasty when the seeds are fully developed so that animals or birds will want to eat them and will possibly carry the seeds further away from the parent plant.

Most of our edible crops are insect pollinated. Bees are the biggest pollinator which is why they are so important. If the flower isn’t pollinated, you won’t get fruit. We are facing a worrisome decline in the population of bees. There are many contributing factors like environmental change, the use of pesticides (even bee friendly ones) or acres of single crop fields. Cities are apparently where bees are thriving, due to the all year round variety of flowers that are grown in the tightly packed gardens. Beekeeping is a difficult and expensive hobby to start in the city and with the population density the possible bee allergies of neighbours could be a concern. However, you can help bees and therefore help your own edible garden by growing bee friendly plants. I’m loathe to grow plants I can’t eat, but here you can …er…have your plant and eat it! There are some edibles that bees love like borage, rosemary, lavender, cornflowers, sunflowers, alliums (I have Chinese chives and three cornered leeks, also Egyptian walking onions but these don’t really flower much), clovers and daisies (I’m trying to grow these in the lawn and next year I’ll find out how edible they are), fennel, beans, mint and nasturtiums. All your other fruiting trees and plants may not be bee favourites, but they do also provide food for them. Other things you can do is to not use pesticides and/or provide homes for solitary bees, which can be something as simple as a log with lots of holes drilled in it.      

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Self-fertile and pollination partners

These are terms you will often hear about fruit trees.

It is in the best interest of the plant to cross pollinate (pollen moves from one plant to another of the same species) as opposed to self-pollinate (pollen moves within the flower or to other flowers on the same plant). It avoids recessive diseases and the more variation a species of plant has, the more likely that at least some of the individuals would survive if a disease or an environmental factor was to wipe out most of its population. This is why some plants have mechanisms that encourage cross pollination. Curcubits (squashes, cucumbers, melons, pumpkins) have separate male and female flowers. There will be times when they don’t open at the same time.

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Some plants are ‘OK’ with self-pollination. This is when fruit trees are called self-fertile. This, at least, means that the plant is more likely to be able to reproduce. Some flowers will have mechanisms that will induce self-pollination if during the day an insect has not cross pollinated it.

Self-fertile trees make life much easier for those of us with small gardens. It means that we don’t have to have a pollination partner – a second tree of the same species but of a different variety that flowers at the same time. With gardens packed together there is a chance that a neighbour might have a pollination partner. This is a good reason to encourage everyone you know to grow food. Often self-fertile plants will perform better with a pollination partner though. Just so you know, two granny smith apple trees will not pollinate each other. All trees of the same variety of an apple are clones. Someone somewhere will have discovered or bred a tasty apple and then called it a variety. Anyone who wants that variety will then have to take a branch of that tree (or a tree that is a clone of the original) and graft it onto healthy and vigorous root to get a tree. Therefore, all named fruit trees are grafted. If you grow a tree from a seed, as discussed above, half of its DNA will be from the original mother tree but half of it will be from another source.

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Even self-pollination gives a little variety. People are not genetically identical to their siblings even though the source of DNA was the same. This is why in ‘Sowing seeds’ I suggest to choose your seeds wisely.