Monocotyledon, or monocot for short, refers to one of two groups of flowering plants, or “angiosperms.” Most flowering plants are traditionally divided into two different categories: monocots and dicots. Members of each group tend to share similar features.
Monocots, as the name implies, are defined by having seeds that contain a single (mono-) embryonic leaf known as a cotyledon. This is a monophyletic group that constitutes a majority of our agricultural biomass and include many important crop staples including, but not limited to, rice, wheat, corn, sugar cane, bamboo, onion, and garlic.
A word of caution: when classifying flowers into monocots or dicots, remember that there are always exceptions to the rule. Some monocots may have a feature typically found in dicots, or vice versa. Even a few flowering plants (approximately 2%) don’t fit into either the monocot or the dicot category.
Features used to Distinguish Monocots from Dicots
Monocots differ from dicots in six distinct structural features. Five of these features are easily observed in the mature angiosperm: the flowers, leaves, roots, stems, and pollen grains. But the root of these differences stem from the very early embryonic stages of the angiosperm, providing the biggest difference of all between monocots and dicots, is the seed.
Flowers usually arrange their parts in circles, with the reproductive parts in the middle surrounded by petals and sepals. In monocots, these flower parts are trimerous. In other words, the flower parts of a monocot are arranged, structured, or numbered in multiples of three—usually with one stigma, three stamens, three petals, and a calyx formed by the sepals in numbers less than or equal to the number of petals.
Veination 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 monocots, these veins are arranged in a parallel-like pattern. Compared to other patterns of veination, in parallel veination, the parallel veins are smaller in size with even smaller veins connecting them.
While monocots start of with a tap root, these tap roots tend to die soon after germination and are replaced by adventitious roots. Adventitious roots look fibrous and are spread widely throughout the soil in many different directions. They tend to occupy the upper layer of soil and can be modified for different purposes like additional anchorage or aerial support. Because adventitious roots typically arise from an organ that is not the root of a plant, such as the stem or sometimes a leaf, we are able to grow multiple plants from stem or leaf cuttings of a pre-existing plant!
It is important to note that the stems of monocots have lost the ability to increase their diameter by producing wood and bark through secondary growth. Instead, monocot stems die down each year, allowing new stems to grow. The only growing point of a monocot stem is at the top of the stem, disallowing the growth of any side stems or branches. Typically, then, monocotyledons are small and herbaceous
In a cross section of a monocot stem, you will find an epidermis, hypodermis, ground tissues, and vascular bundles. Typically, monocot stems have the following characteristics: single layer epidermis with a thick cuticle; lack of epidermal hairs; lack of concentric arrangement; hypodermis is sclerenchymatous; presence of bundle sheaths; oval vascular bundles of different sizes; and most notably, scattered vascular bundles that do not have create any pattern.
Monocots have a pollen structure that is retained from the first angiosperms. The pollen grain of a monocot is monosulcate, meaning that the pollen has a single furrow or pore through the outer layer.
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, monocots are characterized by having one (mono-) cotyledon in the seed, and one leaf emerging from the cotyledon. The seed pod of a monocot is also trimerous (in parts of three), because the carpel from which they grew also consisted of three parts.
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 Monocots
Although we generally don’t think of grass as a flowering plant, they actually do have small flowers that grow at the very tips! The grass family is arguably the most economically important group of monocots. Think of corn, wheat, and rice—they’re all a type of grass whose flowers are often overlooked because they do not have petals or sepals.
The palm tree is an exception to the rule when it comes to monocot plants. Most monocots cannot grow as large and tall as palm trees do because they lack secondary growth—the growth of wood and bark—limiting most monocots to be herbaceous. However, palm trees have circumvented this issue by utilizing their vascular bundles and the lignin within them to create a more firm stem. Palm stems are also thickened by parenchymal cells that surround the vascular bundles, providing even more support for a tall tree-form.
Still, one clearly observable characteristic of a palm tree that identifies it as a monocot is the leaf of the tree. The leaves of a palm tree are long and strap-like, with major veins running parallel to one another.
Probably one of the easiest of all plants to identify as monocots is the lily because it fits the bill for all monocot characteristics. The flowers of most lilies are obvious and trimerous as all three pedals are usually identical in size and shape, the roots are adventitious, it is small and herbaceous, and begins with a single cotyledon.
However, some lilies have developed different petal and flowering structures with may be confusing when trying to classify them as either monocots or dicots. The flowers can come in many different shapes like trumpets, funnels, cups, bells, or even flat shapes. For instance, the flowers of a peace lily is born in clusters on a stem called a spadix, which doesn’t look like your typical flower cluster. A common misconception about the peace lily is that it has just one petal. What most people mistake for the petal is actually a specialized leaf called a spathe. The true flowers on the spadix are, indeed, trimerous.
Similar to the lily, the primary characteristic of orchids that identify them as monocots is the flower. The petals of orchid flowers are clearly trimerous; however, some morphological features distinguishes the orchids from other monocots. Of the petals that grow in multiples of three, one petal has evolved to become a lip, a specialized landing platform for the pollinator. So, in the case of orchids, not all three flower petals look alike, which may be confusing when looking for identifying characteristics of monocots. Also, while the orchid does begin with three stigmatic lobes as is typical with most monocots, they are later fused together, with just a few faint lines to implicate its trimerous structure.
Often incorrectly thought of as a tree, the banana plant is actually a monocot and is closely related to the grass family. As is typical with monocots, banana plants do not have secondary growth; they die down regularly after the banana plant has produced its fruits. The banana fruit even grow in parts of three (tri-locular, having three segments) and have leaves with the familiar pattern of parallel veins.