Date: Summer 2011
Why do some seed plants produce numerous seeds, while others just one?
When a plant produces seed, the purpose is to carry on the species - to produce more plants.
The strategy employed by the plant will vary depending on the environment in which the plant has developed, but in the main there are two main strategies which I categorize in my head as "Special Care" and "Safety in Numbers".
"Safety In Numbers" is how the dandelion chooses to tackle the problem. It produces huge numbers of seeds, and does very little to help those seeds other than to make sure they get away from the parent plant (That's trhe job of the parachute) Other plants which use this strategy include most herbs such as mint, borage, dill where the seeds are very small, and are produced in huge numbers.
The "Special Care" strategy involves giving a huge store of food to the growing embryo, so that it can make a good start, even if the conditions are not ideal. Avocadoes and coconuts are the prime examples. The oil in the avocado seed, and the starchy flesh of the coconut are both provided to feed the embryo plant, even before it is in the ground. The strategy is so effective that coconuts can begin to sprout even when they are still floating in the sea - then when they wash up on a beach somewhere, they are ready to put down roots in a matter of days, and begin growing. Avocados too can begin to sprout when the fruit is still lying on the ground. If things go well, an animal will carry away the fruit and leave the seed behind ready to germinate very quickly - nourished by the food within the seed.
Many plants adopt a strategy which is between these two extremes. An apple tree produces a number of small seeds which have a small amount of nutrient for each seed. An apricot produces only one seed per fruit, but the seed is much larger and has a much larger store of food for the embryo.
Whichever strategy the plant uses depends on a number of factors in play when the particular plant species was developing. Large numbers of seed seem to be more prevelant in environments in which there are pockets of good soil, and little chance of moving once the seed has landed. This seems to apply to many grasses and to trees in arid environments. Larger seeds seem to work best when there is a greater chance of the seed finding a suitable location to grow either because good locations are common (tropical rain forest) or where it is likely that an animal can improve the likelihood of moving the seed to a favorable location.
And then just when you start to see a pattern - you realise that strawberries don't fit either pattern - or are perhaps trying to use both. Strawberry seeds are very small and are found on the outside of the fruit! Those little black specks on the skin of the strawberry are its seeds. They can only germinate after the fruit - and all its seed have been eaten by an animal or a bird, and the hard indigestible seeds are eliminated along with a big blob of instant fertiliser courtesy of the carrier animal.
Isn't nature wonderful.
I hope this is useful
Lab Technician Barkly College - Secondary
Tennant Creek AUSTRALIA
The number of seeds varies depending upon the plant family, and also the production and fertilization of ovules.
The following may be helpful:
Anthony R. Brach, Ph.D.
Harvard University Herbaria c/o Missouri Botanical Garden
Plant species usually produce a characteristic number of seeds that allow the species to survive and expand modestly as a population. Survivorship of a germinating seed depends on many factors that change from year to year and from one place to another. Usually, as the number of seeds made per plant decreases, the amount of investment the plant makes in the seed increases.
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Update: June 2012