This is the season when the leaves fall from the trees, a season I used to love above all the others, but which now tends to bring melancholy reflections on the ebbing of life as one's friends wither and die. But sometimes those reflections bring back rather random memories of moments in a conversation that for some reason have stuck. One such came to me today as I walked through the fallen leaves and noticed the black spots on them. I'm sure you will have noticed them also.
The memory was not of autumn leaves but of spring flowers. My friend and I were undergraduates, he studying English literature and I medicine. As we walked past the daffodils and crocuses leading to the River Cam, I started to talk about plant reproduction. He, his head full of poetry and anxious to talk about Wordsworth (who was then rather out of fashion), asked me if knowing these details didn't spoil one's appreciation of the beauty of nature. And thus started one of those discussions that so characterised those far gone university days when it seemed that rational argument could solve all the world's problems.
It was a time of earnest discussion of the apparent division between science and art, as we all became specialised in one or the other, those on the arts side thinking that we scientists needed a proper education, while we lamented their lack of scientific understanding.
I don't remember how our discussion ended, but I expect it continued after supper in a pub and eventually solved apartheid and the Middle Eastern problems post-Suez. But the initial question has stuck in my head for over 60 years. There is beauty beyond the immediately visible colours, texture and shape of things, and there is beauty in how they work and how they got there. There is poetry in the workings of nature.
Those spotted leaves also took my memories back to the 1970s, during my first job as a consultant physician, seeing patients with an uncommon lung illness called allergic aspergillosis, caused by a fungus called aspergillus. Its name is derived from the aspergillum used to spread incense, because its spores are scattered from a structure shaped like that device. These spores are its reproductive bodies which, if they land in suitable soil, will reproduce their parent organism. They are propelled into the air and are inhaled by humans, but only cause this disease in a small minority of us. I wondered why.
Two facts were known. First, the spores are so small that they can be inhaled deep into the lung and, second, they grow best at a temperature close to ours. But this is true of many other fungal spores in the air and we breathe them in also, yet only aspergillus causes this disease. It took some years to find the answer. The spore releases a chemical from its surface that paralyses lung defence cells, giving it a survival advantage when inhaled. Its problem is that it doesn't reproduce in the lung, so its reproductive purpose is frustrated. Why has it evolved this trick?
Lateral thinking will tell you – the spore's habitat is the soil, not lungs; in the soil, creatures like amoebae eat spores. Evolution gives aspergillus a survival advantage there, and lung defence cells move by the same mechanisms as amoebae. Just as with humans on the Earth's surface, micro-organisms in the soil are competing for food and resources. This competition is also the story behind the discovery of penicillin and streptomycin. In most of us, our lung defences are good enough to remove the spores, but if we are allergic or immunosuppressed, the spore may win and cause disease.
What's this to do with spotted leaves? Well, each of those black spots is a culture of aspergillus, resulting from deposition of a spore on the dead leaf, and there the fungus grows to produce more fruiting bodies and spores to propagate itself – unless it lands on stony ground or in an animal's lung. That understanding surely adds interest to those curling spotted leaves.
