This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
They are opposite or in pairs in the Carnation and Sweet William; in whorls of three in the Oleander, and in whorls of four, six, eight, or a higher number in species of Bedstraw. They are alternate in the Lime, Beech, Elm, and others, where the third leaf comes in a line with the first, counting upwards or downwards. They are spirally arranged on the shoots. This also applies to the Apple, where the sixth leaf comes above the first, after the spiral has passed twice round the shoots. This means that the leaves are separated from one another at an angle of two-fifths the circumference of the stem. These arrangements are intended to distribute the leaves so that all will get a due share of light. This is well shown in the Elm (fig. 38) and the Ivy (fig. 39).
Fig. 35. - Horse-chestnut-Compound Palmate Leaf.
Fig. 36. - Robinia - Compound Unequally Pinnate Leaf.
Fig. 37. - 1, Felted Hairs on Leaf of Edelweiss. 2, Stellate Hairs on the Epiderm of Draba.
Fig. 38. - Leafy Horizontal Twig of an Elm showing Leaves naturally arranged to catch the light.
Not only are the leaves arranged so as to secure the maximum amount of light, but they are also so placed as to contribute to the nourishment of the roots. In the case of such trees as Apple, Pear, Plum, Cherry, Oak, Chestnut, Beech, Elm, Lime, Ash, etc., the leaves are arranged over each other like tiles or slates on a roof to throw the water from the centre to the outside. In this way the rain trickles from one leaf to another, and the great bulk of it falls around the circumference. It is just at this place that nearly all the fibrous feeding roots of such trees exist. Consequently, when the rain falls they get the first supplies, and are thus refreshed and enabled to take up more food from the earth in solution.
In the case of the Yew and many Coniferous trees and shrubs the rain is thrown inwards as well as outwards, owing to the way in which the leaves and branches are arranged.
In the case of other plants, e.g. the Caladium (fig. 40), it will be noticed that the leaves are so arranged that water is thrown to the circumference or periphery of the plant, to which the fibrous roots extend from the tubers. On the other hand, in the case of the Rhubarb (fig. 40) the large leaves directed upwards naturally collect the rain and direct it towards the centre of the plant, and thus down to the thickened rootstock, the fibres from which do not extend horizontally. The leaves of Turnips, Radishes, Beetroot, Parsnips, Carrots, Dandelions, Chicory, etc, and most bulbous plants are arranged in a similar way, to conduct water inwards for the benefit of their rootstocks.
Fig. 89. - Ivy on the ground showing normal arrangement of Leaves to catch the light.