Think of seeds as like the escape pods we might build for our future offspring. If a global catastrophe threatened humanity, we might try to send thousands of smaller space arks to the Moon or Mars. But if the whole Solar System was doomed and we had to reach another star, the best strategy would be to pin all our hopes on a smaller number of brave astronauts in much larger spaceships.
Plants do something very similar. They have evolved seeds with just enough of a protective coat, packed with just enough food supplies, to keep the embryo alive for the time it takes to reach a suitable environment.
Coconuts are the largest seeds (the double coconut can weigh 42kg) and can still germinate after four waterlogged months at sea. Dandelions can produce much smaller seeds because they never need to go far to find a patch of suitable soil. Orchids have the smallest seeds of all (less than a millionth of a gram each) because the chance of finding a suitable spot on a tree branch is very low, so they need to produce millions of seeds at a time.
Once they germinate, the seed also needs to grow enough leaves to be able to survive independently. Plants that live in shady conditions, such as oak trees, produce relatively large seeds because the seedling must grow taller to reach the light.
Evolution must balance many other trade-offs too, however. For example, large seeds are also more attractive to predators. Kangaroo rats in deserts tend to eat the largest seeds, so natural selection in those environments favours smaller seeds that are not worth bothering with.
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