Monday, 12 January 2009

On reading The Origin of Species (ch 4)

Darwin postulates that there may be a 'law of nature' that "a cross with another individual is occasionally - perhaps at very long intervals - indispensable [to increasing 'vigour and fertility']" as I mentioned in my last post.

He goes on to talk about the methods by which flowers may be pollinated - pollination by bees and insects, either by them carrying pollen from the anthers of one flower to the stigma of another, or by triggering some mechanism that causes the flower to self-pollinate - for example by pushing its anthers against its own stigma as the bee or insect crawls into the flower to get at the nectar.

He then goes on to note that in many flowers, such as Lobelia fulgens:

...either the anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower is ready, so that these plants have in fact separated sexes, and must habitually be crossed. How strange are these facts! How strange that the pollen and stigmatic surface of the same flower, though placed so close together, as if for the very purpose of self-fertilisation, should in so many cases be butually useless to each other! How simply are these facts explained on the view of an occasional cross with a distinct individual being advantageous or indispensable!
He also says two things that seem somewhat at odds - and he hints at further coverage of the points later on. First off:
...if you bring on the same brush a plant's own pollen and pollen from another species, the former will have such a prepotent effect, that it will invariably and completely destroy, as has been shown by Gartner, any influence from the foreign pollen.
Then a few paragraphs later:
Yet the pistil of each cabbage-flower is surrounded not only by its own six stamens, but by those of the many other flowers on the same plant. How, then, comes it that such a vast number of the seedlings are mongrelised? I suspect that it must arise from the pollen of a distinct variety having a prepotent effect over a flower's own pollen... appears to me that the principle is acting one way in one case and in another way in a different case - almost as if nature knows which organisms are distinct species, and which are distinct varieties....

No comments:

Post a Comment