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.
Seed leaves are always simple or in one piece, though they are lobed in a few cases. Simple leaves (fig. 33) are represented by those of the Cherry, Apple, Fuchsia, and Camellia. They may be more or less deeply and palmately lobed, as in the Vine, Ivy, Sycamore, Plane, and Hop. The lobing may be in the form of a feather, as in the common Polypody, Marguerite, Oak, and Water-cress. This form is termed pinnatifid. The cutting is carried still deeper in Celery, Parsnip, and Carrot, and the simple leaf termed pinnatisect, or it may be twice pinnatisect in the Carrot. Leaves are "compound" when each separate piece into which they are divided is jointed, as in the Laburnum (fig. 34), the Virginia Creeper, Horse-chest-nut (fig. 35), and Aralia Veitchi. The latter three have palmate leaves. The feathered type of compound leaves is termed unequally pinnate in the Rose, Robinia (tig. 36), Ash, Elder, and Walnut. The Laburnum and Clover have ternate leaves, because cut up into three jointed leaflets. The forms of leaves are practically endless, and should be studied from the textbooks.
Fig. 33. -Simple Leaf.
p, Petiole, with Stipules at the base; 1, midrib; 2,3, branches of the midrib.
The surface of leaves may be smooth or glabrous, that is, without hairs, or they may be covered with hairs varying greatly in density, length, or form. Hairs that lie smooth and close are silky; those that are interwoven with one another are felted or tomentose (fig. 37); long and loose ones may make the leaves shaggy or woolly. Amongst the hairs on the leaves of the Nettle some have a swollen base and are stinging. Those of the Stock are branched in a starry fashion and are termed stellate. To this class belong the hairs on Draba (fig. 37), while this is carried further in Elaeagnus and other shrubs, the branches of the hairs being united in the form of circular scales, like a Japanese parasol in miniature, with a very short stalk. The hairs are useful in a variety of ways, by running the moisture off the plants, by preserving the liquids in the leaves of plants that live in dry places; while the hairs and bristles on many Cacti serve to keep them cool under the influence of a scorching sun in desert regions.