The Lives of Leaves

Forest and Bird, Issue 310, 1 November 2003, Page 40

The Lives of Leaves

ANN GRAEME

ANN GRAEME

looks at what leaves do.

Photographs, GORDON ELL.

leaf is a factory for making food. It food for the plant, and the plant in turn is food — for animals, fungi and bacteria that haven't got leaves and therefore can’t make their own food. So, leaves are much more than just an abstract interest to us animals! The leaf-factory is efficiently designed. It has a transport system to bring in the raw materials, water from the roots along the xylem vessels or pipes in the leaf veins, and carbon dioxide from the air through the portholes in the leaf called stomata. The food-making machinery is the palisade cells, which contain the chloroplasts. The sun is the source of energy that powers the leaf-factory and the leaf is angled to catch the sunlight. To enable the chloroplasts to function efficiently, the palisade cells are packed along the upper, sunny side of the leaf. In the chloroplasts the raw materials of water and carbon dioxide are combined in a chemical reaction which yields sugar, oxygen and water. The secret of the process, the spark that makes it happen, is the green pigment, chlorophyll, contained in the small chloroplast bodies. The manufactured sugar is then dispatched through the phloem tubes in the leaf veins around the plant to the stems, roots and growing buds. Food-making is the primary function of the leaf; so, all things being equal, the best food-producing leaf would be large, thin and green. But all things are not equal in the natural world. The friendly sun, the source of energy, can also be the plants enemy, evaporating so much water out of the stomata portholes that the plant wilts and dies. The drying wind sucks out water too, and wind and rain and hail hammer the soft leaf and damage its machinery. So leaves must compromise. They are modified in a host of ways to cope with the environment while still carrying out their food-making functions. Big, soft leaves like those of the whau grow in cool, sheltered forests. Leaves in windswept places like rangiora, or exposed to burning salt spray like

pohutukawa, are often leathery and may have a waxy covering on the upper surface and a downy coat of hairs underneath the leaves. This is smart. The waxy upper surface protects the leaf from damage and seals in moisture but still lets in the light. But being impervious to water precludes having stomata on the top of the leaf so they are on the undersurface amongst the hairs. There they can be open to admit air but lose little water in their shady, hairy shelter.

High up in the mountains where the sunlight is intense and the temperatures freezing, leaves may be woolly-coated all over like the native edelweiss. The young, tender leaves and shoots of alpine plants are often coloured with red pigments known as anthocyanins, which acts as a sunscreen. Just as the wide, flat leaves of the dicotyledons are adapted to their environment, so too are the spear-like leaves of the monocotyledons. In harsh environments they may have a waxy or

hairy surface, and some tussock grasses have curled blades so that the vulnerable stomata on the undersurface are wrapped inside. It’s not just the weather that threatens the leaves. Countless animals eat them. Losing all their food factories would kill the plant so there are many ingenious adaptations for defence. Leaves may be thorny or have stinging hairs, or be loaded with poisonous by-products to make them unpalatable.

All leaves have a ‘use-by’ date. The leaves of deciduous trees develop in the springtime and fall off in the autumn. As they mature to old age they often accumulate unwanted chemicals which are shed when the leaves fall. These chemicals cause the brilliant reds, oranges and yellows of autumn leaves. Evergreen trees follow a less rigid pattern. Some leaves age and are shed throughout the year, but leaves can live and function for

several years. Pohutukawa is one of the trees which keeps individual leaves for several years, so if possums continue to chew off the new shoots the pohutukawa will in time run out of replacement leaves and die. Outside the concrete canyons of the city, we are surrounded by leaves. They cover the lawn, the garden, the fields of grass and

crops and the forests. They are the green wallpaper of our world and it is easy to overlook them, so it is good to remind ourselves how much we owe to the humble leaf.

is national co-ordinator of Forest

and Bird’s Kiwi Conservation Clubs. She lives in Tauranga.