Flowering plants, or angiosperms, have traditionally been divided between two different classifications: Monocots and dicots. Although there are some monocot plants exhibiting dicot characteristics and vice versa, the classification system holds true for most all flowering plants and has been used for taxonomic purposes as far back as 370 BC when Theophrastus first documented the differences. The following are the main differences between monocots and dicots.
The first difference between monocots and dicots is in their seed structure. Monocots, as implied by its name (mono meaning one), have a singular seed, like corn. Dicot seeds can be divided into two, like peas. Once the seed sprouts, monocots will produce a singular first leaf (known as a cotyledon), while the dicots will unfurl two leaves.
As the plant continues to grow, we see more difference. When a monocot flowers, it will have petals in multiples of three. Dicots have petals in multiples of four or five. Think of lilies (monocots) and sunflowers (dicots). There is also a difference in their leaf structure. Monocot leaves generally run parallel with the length of the leaf; while dicot leaves produce auxiliary veins that branch out from a main vein. Think of grass blades versus oak leaves.
Differences also occur under the ground, in the plants' root systems. Monocots produce roots adventitiously, where they grow in a cluster from the bottom of the plant. Onions display a good example of monocot roots. In dicots, the roots develop from a single region known as the radicle, often forming a taproot.
Within the stem of the plant, we find its vascular bundles. This refers to the function in the stem that allows for transportation of water and nutrients throughout the plant. In monocots, the vascular bundles are scattered, where in dicots, they form rings. You will see the difference if you cut through a plant's stem and look at the cross section. Another main difference between the two is that generally, monocots will not produce secondary growth. Most dicots, however, have the ability to continue diameter growth, sometimes forming woody stems.