Plants: A Very Short Introduction by Timothy Walker

Author:Timothy Walker
Language: eng
Format: epub
Publisher: OUP Oxford
Published: 2012-03-28T04:00:00+00:00

9. Lodoicea maldavica has the largest seeds in the world. It grows only on the Island of Praslin

Seeds vary greatly in size. The seed of orchids is tiny, almost dust-like, whereas the seeds of the double coconut, Lodoicea maldavica, is the size of two Aussie Rules footballs; that is, a variation of seven orders of magnitude in weight, and yet both the orchid seed and the coconut seed fulfil the same function, and both have the potential to produce a mature flowering plant. Even on the same plant, seeds can vary in size by three orders of magnitude, and the seeds within the same fruit can be different. One rule of thumb seems to be that plants that live in shade tend to have seeds from the bigger end of the scale, perhaps as this enables them to become established in an environment with lower energy levels. Seed size appears to be linked to neither the moisture content of the soil nor the nutrient status of the soil. It may just be that by producing seeds of various sizes, the plant is hedging its bets and increasing its chances. One size of seed does not fit all situations.

In the previous chapter, we saw how many plants have come to rely on a stranger to distribute their sperm, and those who do not trust an animal have thrown caution literally to the wind. We know that neither biotic nor abiotic pollination methods are totally satisfactory because hand pollination of a plant is nearly always more successful than leaving it to nature. Some families of flowering plant are particularly hopeless in this respect. Only up to 7.2% of flowers in the Protea family are successfully pollinated, though far more are visited by the pollinator. It may be that their chosen pollinator is now extinct. (Plant species have a general problem when forming joint ventures with animal species, and this problem is that plant species last 30 times longer in evolutionary terms than animal species. So if you are going to use an animal to distribute your pollen or your seed, you will have to be prepared to change suppliers repeatedly.)

We see in plant reproduction a good example of the Allee effect, namely that it is better for the members of a species to hang around with each other – the scientific safety in numbers principle – because the fewer plants and flowers there are, the fewer successful pollinations there are, and so the fewer seeds are produced. Before you start to think there is a rule appearing, you have to remember that although the bigger groups of inflorescences attract more pollinators and so produce more seeds, they also attract more flower grazers, more seed predators, and more fruit grazers. So there is no optimum size for seeds; the best size is time and space specific, and so the best strategy is variation.

Charles Darwin was not only interested in pollination, he was also fascinated by the geographical distribution of species. He carried out

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.