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.
The first leaves of a plant are those formed in the seed, and which may or may not rise above-ground during germination. Those of the Cabbage, Mustard (fig. 13), Gourd (fig. 18), Beech, and Onion rise above-ground and become green. As soon as this has taken place the seedling has started life on its own account, manufacturing its own food in the green seed leaves and stem. The seed leaves of many plants become fleshy, store food while in the growing seed, and never rise above-ground during or after germination, but supply food to the seedling till able to forage for itself. Examples of this may be seen in the garden Pea, Sweet Pea, Broad Bean, Scarlet Runner, Horse-chestnut, and Oak (fig. 19, p. 35). The seed leaves differ more or less widely in form from the true leaves that follow. All of the plants mentioned in this paragraph, except one, have two seed leaves or cotyledons and belong to the class Dicotyledons. The Onion, Lily, and others have only one cotyledon, and are Monocotyledons. The seed leaves of Iris and grasses never rise above-ground, and as they have only one each they belong to the latter class.
The true or rough leaves are those that follow the seed leaves in succession, increasing in size and varying in form with each individual till the plant reaches the adult state. The leaves, taken altogether, constitute the foliage of the plant.