Autumn has finally arrived, and in quite a rush. There is some rain, for which all gardeners are truly grateful, still some sun (though the air itself has the requisite seasonal chill morning and evening), and the colours are starting. Perhaps a little investigation of that phenomenon is in order.
What makes leaves "turn"? In a word: "anthocyanins". Or, at least, that works for leaves that turn red or purple, and the process is more complex than was once thought. There is evidence that the red-spectrum colourings are a result of the breakdown of chlorophyll in leaves towards the end of the growing season, but it has also been suggested that the red colouring signals an increase in toxicity just before leaf-fall (which might itself prompt the question: why then?), or another form of protection: against sunlight itself, or as an aid to the reabsorption of nutrients into the body of the plant concerned before it goes dormant for the winter.
Orange and yellow colouring, caused by different chemicals called carotenoids, seems to have a rather different origin: the seasonal breakdown of chlorophyll that occurs when plants "go to sleep" reveals colour pigments previously hidden by the green colouring of growing leaves. The intensity of leaf colour change can be affected by many factors, including maximum-minimum temperature difference, humidity, and the amount of water in the soil.
Evergreen trees also shed their "needles" in autumn, the old ones turning yellow or brown before they fall. Since it is only a proportion that drops, this is much less noticeable than the deciduous shedding.
On a related issue: why are so many berries red? It's caused by similar chemicals, but the bright colours simply grab the attention of birds and other animals in need of a meal during the testing winter period. Red equals health and ripeness, and I can personally attest to the attraction of yellow/red crab apples to gangs of marauding (and messy) wood pigeons. Nonetheless, white and black berries (ivy and mistletoe, for example) will also be consumed eventually, even without such colour signals - food will be scarce, and the animals and birds still have to eat! By the way, leave ivy and mistletoe to them - some hips (and haws) are edible by humans, ivy certainly isn't, and, though druids are said to have used mistletoe to induce trances, it's dangerous stuff!
In spite of the extremely dry summer we have just experienced, trees with berries and other fruit seem absolutely loaded, to the extent of broken branches in allotments and orchards. I have already tried some locally-grown greengages and plums, which tasted great, while on a recent walk north of the village I saw hundreds of acorns and wild apples in great abundance. If schoolboys are still interested in such things, I also gather that conkers are disappointingly small this year - how will they ever manage, if not too glued to the screens of their phones?