Flowering plants add grace and beauty to our world. More pragmatically, they give us food, clothing, and shelter too. Life as we know it depends on angiosperms.

The two most significant divisions of angiosperms are the monocots and dicots. Recently, molecular geneticists have suggested that the more primitive members of the dicots should be split off and the remaining group renamed the eudicots (true dicots), but this proposal hasn’t yet gained widespread support. Even with the suspect members removed, the eudicots still comprise 75% of angiosperms, and 50% or more of all plants.

There are a number of structural differences in the roots, stems, leaves, flowers, and pollen that distinguish monocots from dicots. There are exceptions to every characteristic, but two or three are usually sufficient to correctly place a plant.

Embryos
Oddly enough, the namesake difference between these two great plant groups is the most transient and least easy to see. A cotyledon, or seed leaf, is part of the embryo of a seed plant. While the embryo is inside the seed, the function of the cotyledon is to absorb and in some cases store food from the surrounding starchy tissue.

Monocots are monocotyledonous – they have 1 cotyledon. In most of these plants, there is still food stored in the seed at the time of germination, and the cotyledon continues to take up nutrients and pass them on to the rest of the seedling until it is able to photosynthesize.

Dicots are dicotyledonous – they have 2 cotyledons. In most dicots, the cotyledons have absorbed all the nutrients in the seed by the time germination occurs. Some will feed the seedling until it can photosynthesize, and then wither and fall off. Others turn green and start to photosynthesize themselves.

Roots
In most seed plants, including dicots, the root grows from the lowest part of the embryo, a region called the radicle. A dominant central root, called a tap root, grows downward, branching off lateral roots that spread horizontally. Plants with this system are able to reach more deeply into the soil to find water. The tap root also has a food storage function, and many root vegetables like carrots and radishes are swollen tap roots.

In monocots the radicle aborts and roots arise from multiple points on the stem. Called adventitious roots, they form a tangled network that holds the plant firmly in the soil.

Stems

Vascular tissues – the long tube-like cells that transport water, nutrients, and food inside plants – are arranged in bundles. In monocots, the bundles are scattered throughout the stem. In dicots, the vascular bundles occur at regular intervals that form a circle around the periphery of the stem.

Because the vascular tissue in dicots is arranged in a ring, the stems can grow in diameter by adding new layers of xylem and phloem to the ring. This expansion is called secondary growth. Since xylem cells have stout, reinforced walls, secondary growth adds strength and well as girth to a stem and allows dicots to grow quite large.

The random arrangement of vascular bundles in monocots precludes true secondary growth. Other stem tissues can multiply or expand and widen the stem to some extent, but they lack the strength of xylem. Additional vascular bundles can be added to the margin of the stem, but these also don’t provide as much support as a continuous wide band of xylem. Monocot trees do exist, such as palms and bamboo, but their trunks tend to be relatively slender.

Leaves
The main veins in monocot leaves are parallel to one another and generally equal in size. Many have elongated, even blade-like leaves. Because monocot leaves grow outwards from the base, they can resume growing after the tip is grazed or mown.

Dicot leaves have net-like vein systems, where a patchwork pattern of small veins fills the area between large, branching, divergent veins. Dicot leaves grow from the edges, so parts of a leaf that are bitten or torn away will not regenerate.

Flowers
Flower parts such as the sepals, petals, and stamens occur in multiples of 3 in monocots. In most of them the total for each part is 3 or 6. The flower parts of dicots come in multiples of 4 or 5.

This feature is unreliable in flowers that have either reduced or very numerous parts.

Pollen
The final and most consistent difference between monocots and dicots is in their pollen. Monocots have a single pore or furrow on the outside of their pollen grains, while dicots have three.