Dicotyledons

Dicotyledons are characterized by the presence of two seed leaves (cotyledons) in the embryo contained within the seed. In contrast most monocotyledons have only one seed leaf.

The angiosperms are a large group of plants that, unlike gymnosperms, bear flowers and have seeds that are completely enclosed. In many modern systems of plant classification, the angiosperms (subphylum Magnoliophy-tina) are divided again into two classes: the dicotyledons (Magnoliopsida) and the monocotyledons (Liliopsida). (This classification, however, may vary. See The World Book Encyclopedia article flower [How flowers are named and classified].) The major basis of these groups, and the one that gives them their names, is their number of seed leaves: dicotyledons have two seed leaves and monocotyledons have one.

There are more than 190,000 species of dicotyledons and all of them, from garden flowers, such as hollyhocks [Althaea spp.), to complex trees, such as oaks (Quercus spp.), have within their seeds two seed leaves or cotyledons. These may remain underground during the germination process or appear above ground. When visible, the cotyledons have a simple, usually rounded shape.

Magnolias (Magnolia spp., left) are considered to be among the most “primitive” (least highly evolved) genera of dicotyledons. The parts of the flower are numerous, unjoined, and arranged in whorls. The wallflower (Cheiranthus sp., right) is intermediate in evolutionary development, having flowers with four unjoined petals, four sepals, six stamens, and one stigma.

Leaves, shoots, and roots

Apart from two seed leaves, dicotyledons have other distinctive anatomical features. Their true leaves (those other than the seed leaves) have veins, usually arranged in a netlike pattern with a distinctive central vein, or midrib, and the base of the leaf usually tapers to a point.

The stems of dicotyledons also have characteristics that typify the group: the water-conducting cells (xylem) and those that transport dissolved food substances (phloem) are grouped in open vascular bundles arranged around the perimeter of the stem. Between the xylem and phloem is a growing layer (cambium), which produces new xylem and phloem cells in woody plants (shrubs and trees).

Another distinctive feature of most dicotyledon stems is that, as their complement of xylem and phloem cells increases through the activity of the cambium, the stem becomes toughened or thickened. If this process-known as secondary thickening—continues, the stem becomes woody.

Like their stems, the roots of most dicotyledons also contain xylem, phloem, and a cambium capable of creating secondary thickening. Whereas the water-conducting xylem tissues of monocotyledons are commonly arranged in a ring, those of a dicotyledon root take on a characteristic cross or star shape.

Flowers

Monocotyledon flowers, such as tulips (Tulipa spp.) and lilies (Liliaceae), have their parts arranged in threes, whereas those of the dicotyledons have a different, more varied numerical formula of construction. Most dicotyledons have flowers with their parts—including petals, sepals, the pollen-producing stamens and the pollen-receiving stigmas, and the parts of the ovary in which embryo seeds are contained—grouped in fives or fours. In the dicotyledon flowers, which are thought to have evolved first and which are therefore the most “primitive,” the parts are numerous and are arranged in whorls, not joined together. The composites—that is, daisies and their relations—have the most specialized dicotyledon flowers, made up of hundreds of tiny florets.

Dicotyledon flowers are structurally quite varied, the differences in petal structure being one of the most significant features. Plants whose flowers have free (unjoined) petals, such as roses (Rosaceae) and buttercups (Ranunculus spp.), and those with no petals, such as those of many trees, including the birch (Be-tula sp.), are placed together in one group, the Archichlamydeae. Those with petals completely or partly fused, such as gentians (Cen-tiana spp.l, bluebells (Campanula spp.), and heathers (Erica spp.), belong to a second group called the Metachlamydeae.

The two cotyledons (seed leaves) of a seedling kidney bean appear above the ground. A cross section of the bean’s stem reveals, as in most dicotyledons, vascular bundles (groups of xylem and phloem cells) arranged in a ring around the perimeter of the stem. These cells are separated by a growing layer called the cambium. The root of a typical dicotyledon has its xylem and phloem cells grouped at the center—the xylem arranged in a cross or star shape, with the phloem between it.

Distribution and diversity

Dicotyledons have evolved to occupy all kinds of habitats, from tropical forests to high mountains, from deserts to fresh or salt water. Only being blanketed with a permanent covering of snow and ice acts as a total deterrent to species of dicotyledons.

In their life styles, dicotyledons show an equal diversity. At the extremes, for example, they may be “insect-eaters,” such as the sundew (Drosera sp.), or pale parasites, for example, the dodder (Cuscuta sp.), which absorbs its nutrients from the stems of other plants.

Dicotyledons may be perennials, biennials, or annuals. In some harsh environments, annuals grow, bloom, and die in the space of a few weeks. The perennial species may be herbaceous—dying down to ground level each year—or woody, with a permanent growth structure above the soil level.

Leaves of dicotyledons vary widely in shape, size, and complexity. Undivided (simple) leaves include the linear campion, the peltate nasturtium, and the palmate maple leaf. Typical compound leaves (made up of several leaflets) include the pinnate vetch leaf and the palmate horse chestnut leaf.

The dicotyledons include many of the most prolific wood- and food-producing plants. All the world’s hardwoods, from oak to teak, come from dicotyledons. Of the many plant families that provide food for human beings and their domestic animals, the following dicotyledons are among the most significant: the Cruciferae, which includes all of the cabbage family; the Solanaceae, to which potatoes and tomatoes belong; the Rosaceae, from which come apples, pears, and other soft fruits; the Compositae, whose members include the chicories and lettuces; the Cucurbitaceae, the family of melons, marrows, and cucumbers; and the Leguminosae, from which come the beans and peas.

The inflorescence of the daisy (Beilis sp.) consists of many florets. In such a highly evolved flower, only the central disk florets are capable of forming seeds. The purpose of the outer, petallike florets is to attract pollinating insects and birds.