The Function of Plant Leaves
Much of the beauty in a garden is provided by the colour, form and texture of the leaves of plants. But this beauty is a consequence of and is subordinate to function, for within the leaves lie the food factories of the plants.
The great function of plant leaves is a unique chemical process known as photosynthesis — literally, manufacture by light. The leaves have the ability to use the energy of light to make sugars and other complex foods from the simple raw materials of the water solution absorbed by the roots from the soil and of the carbon dioxide absorbed by the leaves from the air. These sugars and foods serve to feed the plants themselves and, in turn,’ almost all other living organisms which depend on plants for their food.
Photosynthesis is a process which no chemist has yet been able to simulate. Without it neither plant nor animal life, as we know them, could exist.
The key to the process is the substance contained in the leaves that makes them green — chlorophyll. This substance absorbs the light rays, chiefly the red, blue and violet, and converts them into a form of energy for the manufacture of the plant’s food.
The arrangement, disposition, shape and structural characteristics of a plant’s leaves are the result of its evolution in its native habitat, and are designed to enable the plant to carry out photosynthesis and related processes with the maximum efficiency.
Leaves are of many sizes, shapes and textures. The larger the leaves, the fewer the plant has of them. Magnolia obovala may have leaves up to 18 in. long, while those of a heath, such as Erica camea, are only 1/4 in. or less.
Leaf shapes vary astonishingly. Botanists divide them broadly into simple, single leaves, like those of a cabbage, compound leaves like those of a clover, palmate like those of the horse chestnut and pinnate like those of the ash.
They further split them into four series — oblong, with more or less parallel sides; elliptic, with curved sides tapering equally to base and tip; ovate, with curved sides like an oval and widest below the centre; and obovate, with curved sides, widest above the centre.
Each series is further graded according to the ratio of length to breadth, and leaves may be smooth-edged, toothed, lobed or divided.
The shape of the leaves and their disposition on the plant is part of the plant’s solution to the problem of securing the maximum exposure of leaf surface to light and air. If a plant is looked at from above and from all sides, it will be seen that it makes the best possible arrangement of its leaves on their stems to ensure that the leaves receive the most light. This arrangement is known as leaf mosaic.
The leaves, branches and shoots of a plant are arranged in any one of three ways. The leaves may grow in opposite pairs, with each new pair produced at right angles to the previous one, as in the ash or lilac, or the leaves may grow in whorls of more than two leaves at the same level. Each whorl turns sufficiently to allow its leaves to grow directly above the spaces between the leaves of the whorl underneath. In some plants the whorl is a rosette of leaves just above ground level, as in the daisy. The commonest arrangement is spiral, where the leaves alternate in two or three rows in their appearance on the shoot, like a circular staircase. This is the pattern of arrangement adopted by many shrubs and trees.