I have an oak tree in a pot grown from a seedling. After three years it is healthy but barely a metre tall. However, its leaves are much the same size as those of a mature oak. In animals, a stricter law of proportion seems to apply: babies have tiny hands and kittens tiny claws. Why this difference?
鈥� Infant paws and kitten claws are mechanical tools and must be of a size, and leverage, appropriate to the power provided by the limbs.
Rather than hands, perhaps it would be more appropriate to compare leaf size with the diameter of the hair on a child鈥檚 head or the fur on a kitten, which doesn鈥檛 greatly change as the animals mature.
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Or you could compare leaves with mitochondria. Both provide power, and neither changes in size as the organism grows, they just increase in number.
Peter Urben
Kenilworth, Warwickshire, UK
鈥淭he amount of light does have some effect on leaf size, and this is used in the Japanese art of bonsai鈥�
鈥� The function of leaves is different to that of hands or claws. Trees use leaves to gain nutrients by photosynthesis and energy from respiration.
As a tree grows, it needs an ever-increasing surface area to perform these functions. But the leaves don鈥檛 have to get bigger, the tree just makes more of them. A study in 2012 that a mature oak has around 230,000 leaves.
Anne Campbell
Cardiff, UK
鈥� Since a tree leaf has a low weight compared with the rest of the plant, it isn鈥檛 a problem for a sapling to grow full-size leaves, unlike hands on a baby. And the law of proportion in animals isn鈥檛 so strict: a baby鈥檚 head is proportionately bigger than an adult鈥檚. This is mainly so that the brain can be bigger than would otherwise be the case, enabling the baby to learn more quickly.
The amount of light does have some effect on leaf size, and this is used in the Japanese art of bonsai. Here, suitable tree varieties (often those with naturally smaller leaves) are kept miniaturised. Leaf size is kept small by a combination of bright light and removing any new leaves that grow in the spring; the tree then grows another set of smaller leaves. This unnatural process puts a large strain on the plant, so needs much skill by its owner to produce the desired appearance without killing it.
Richard Swifte
Darmstadt, Germany
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