Dicotyledon, or dicot for short, refers to one of two main groups into which flowering plants (angiosperms) are categorized. Most flowering plants are traditionally divided into two different categories: monocots and dicots. Members of each group tend to share similar features.
Dicots, as their name implies, are named for the number of cotyledons, or embryonic leaves, found in the seed embryo—they have two (di-) cotyledons. Unlike monocots, dicots are not a monophyletic group—meaning that the evolutionary history of dicot plants cannot be traced to a single most common recent ancestor. Instead, a number of lineages diverged earlier than the monocots did.
A word of caution: when classifying flowers into monocots or dicots, remember that there are always exceptions to the rule. Some of the early-diverging dicots seem to have typical monocot characteristics such as scattered vascular bundles, trimerous flowers, and monosulcate pollen grains. Some flowering plants (approximately 2%) don’t fit into either category.
Features used to Distinguish Monocots from Dicots
Dicots differ from monocots in six distinct structural features. Five of these features are easily observed in the mature angiosperm: the flowers, leaves, roots, stems, and pollen grains. However, the root of these differences stems from the very early embryonic stages of the angiosperm, providing the biggest difference of all between monocots and dicots: the seed.
Flowers usually arrange their parts in circles, with the reproductive parts in the middle surrounded by petals and sepals. In dicots, these flower parts are pentamerous. In other words, the flower parts of a dicot are arranged, structured, or numbered in multiples of five, or sometimes four. This is not reliable, however, and is not the easiest characteristic to look for in flowers that have either reduced or numerous parts.
Venation refers to the pattern of veins in a leaf blade. These veins are responsible for the transport of water and carbohydrates throughout the plant. In dicots, these veins are arranged in a net-like, or reticulated, pattern. The veins in such leaves appear to look like a finely branched network throughout the leaf blade, with finer veins reticulating between the major veins. However, as with the number of flower parts, leaf venation is also an unreliable characteristic upon which to base your classification assessment. Some flowers may display reticulated venation but are actually monocots such as the aroids and dioscoreales families.
In plants, the radicle, or the embryonic root, is the first part to emerge from the seed. It shoots down into the ground and begins taking up nutrients and water from the soil. The radicle of a dicot plant develops into the root of the plant. More specifically, the root of a dicot is known as a tap root. Tap root systems have a long and deep primary root, with smaller secondary root growths laterally branching off of the primary root.
Dicots exhibit secondary growth, which is the ability to increase their diameter via the production of wood and bark. This is the result of two lateral meristems: the cork cambium and the vascular cambium. These lateral meristems continue to produce new cells throughout the life of the woody dicot plant, ultimately increasing the girth of the plant. The rigidity of wood and bark provide mechanical support against gravity and desiccation to dicots, allowing them to grow large, tall, and solid.
In a cross-section of a dicot stem, you will find an epidermis, hypodermis, endodermis, ground tissues, and vascular bundles. Typically, dicot stems have the following characteristics: multicellular epidermal hairs all over the epidermis; chollenchymatous hypodermis; pith; differentiated ground tissues; and a limited number of vascular bundles in a concentric arrangement. The vascular bundles are typically limited to numbers of four or eight, and arranged near the perimeter of the stem in one or two rings.
Pollen grains are like the male sex cells of plant; they are the male gametophytes that produce the plant’s sperm cells (male gametes). The pollen grains of dicot plants have physical characteristics that distinguish them from the pollen grains of dicot plants. If you look closely, dicot pollen grains are tricolpate, meaning that they have three ridges that go through the outer layer. This structure is derived from the first angiosperms, which had monosulcate pollen grains (having one ridge). The monosulcate form, however, was not retained over the course of divergent evolution.
The plant embryo is the part of the seed that contains all of the precursor tissues of the plant and one or more cotyledon. As the name suggests, dicots are characterized by having two (di-) cotyledons in the seed, and two embryonic leaves emerging from the cotyledons.
The seed pods of a dicot are variable in size, shape, texture, and structure. Dicot seed pods can have almost any number of chambers, including zero. More often than not, dicot seed pods contain more seeds than a monocot seed pod.
The cotyledon is the first part of the plant to emerge from the seed, and is the actual basis for distinguishing the two main groups of angiosperms. Cotyledons are important in food absorption and are responsible for absorbing nutrients from the environment until the plant can photosynthesize its own nutrients.
Examples of Dicot
Although we generally don’t think of these trees as flowering plants, they do in fact have insignificant, inconspicuous flowers. These flowers are often overlooked because they are small and yellow-green, often just blending in with their surroundings.
Perhaps a more obvious indication that the oak tree is a dicot is the presence of wood and bark as a result of secondary growth—a characteristic not found in monocots. In fact, all true trees that have wood and bark are dicots, including maple trees, apple trees, and sycamores. Less obviously, their roots do develop from the radicle, which is typical of dicot plants.
The daisy is an herbaceous plant without secondary growth. Normally, secondary growth is a tell-tale sign of a dicot, but the daisy’s lack of secondary growth does not mean it is not a dicot; in fact, only about half of dicot species are woody. Instead of using the presence or absence of secondary growth to determine whether the daisy is a dicot, there are other characteristics that you can look for. If you counted all the petals on a daisy, you would find that the daisy’s floral parts appear in multiples of four or five, which is characteristic of a dicot. Additionally, the leaves of the daisy have veins that branch in a net-like, or reticulated, pattern—also telling us that the daisy is a dicot.
Rosa is a genus containing over 100 species of perennial shrubs in the rose family, and roses are, in fact, woody. In the wild, roses usually grow five petals, consistent with the pentamerous pattern seen in all dicots. When cultivated in gardens, however, roses can have double or triple the number of petals while still keeping with the pentamerous pattern. The leaves show a reticulated pattern of venation and the cross-section of a rose stem would reveal concentrically arranged vascular bundles.
When we think of flowering plants, cacti may not be the first or even second thing to come to mind; however, cacti do have flowers that bloom under just the right conditions. And when they bloom, their flowers are large, showy, and pentamerous (although it may be difficult to count because in many genera, these flowers have dozens of petals and stamens)!
The leaves of cacti are modified to reduce water loss, making it difficult to check for venation pattern. However, the root systems of the cactus plants have not been modified: they have retained the ever-familiar tap root system characteristic of dicot plants.
Peas, beans, lentils, and peanuts are all dicots with flowers that grow in clusters. It may be difficult to count the number of floral parts, but they do appear in multiples of four or five. Stems of legumes vary in woodiness and size, but their leaves are all distinctly reticular.