Figure 17-3Monocots and eudicots are two major groups of flowering plants that differ in several structural features.

The flowering plants are classified into two major groups, based on some prominent structural features of their seeds, leaves, stems, flowers, and roots. The names of the groups derive from a structure in the plant embryo within the seed, called a cotyledon (pronounced-COT-uh-LEE-din), which usually becomes the first embryonic leaf or leaves of the plant. Plants in which one cotyledon forms are called monocots, and plants in which two cotyledons form are called dicots.

Unlike the monocots, the dicots are not a monophyletic group and include many different lineages. (Recall from Chapter 10 that a monophyletic group is one in which all of the individuals are more closely related to each other than they are to any individuals outside that group.) However, a large subset of the dicots, called the eudicots (pronounced you-DIE-cots), is a monophyletic group.

In addition to having seeds with only one cotyledon, the monocots (about 70,000 species) differ from the eudicots (just under 200,000 species) with respect to four other common structural features (FIGURE 17-3).

Figure 17-4Vascular tissues of monocot and eudicot stems. Cut through an asparagus spear and you can see the vascular tissue (shown here in purple), which is arranged in bundles and scattered throughout its stem.

• 1. Leaves. Monocots generally have parallel veins in their leaves, while eudicots have branching veins.
• 2. Stems. Within the stems of monocots, the vascular tissue—the tissue responsible for nutrient transport—is arranged as numerous, randomly scattered bundles. You can see the scattered vascular bundles if you cut through an asparagus spear (FIGURE 17-4). In eudicots, such as a sunflower, the vascular tissue is arranged in an orderly ring.
  
• 3. Flowers. The flower parts (such as petals) of monocots typically occur in multiples of three, while in eudicots they occur in fours or fives.
• 4. Roots. Monocots typically have many fibrous roots branching from the stem. Eudicots usually have a taproot, a single primary root with smaller roots branching from it.

Some common monocots are palm trees, orchids, lilies, and all of the grasses, including most of the grains used in food products. Common eudicots include roses, daisies, coffee, potatoes, apples, pears, peaches, strawberries, and most large trees, including maples and oaks.