Monocotyledons, also known as monocots, are grass and grass-like flowering plants (angiosperms) with only one embryonic leaf, or cotyledon, in their seeds (Lilianae sensu Chase & Reveal). They are one of the major groups into which flowering plants have traditionally been classified; the rest of the flowering plants with two cotyledons are classified as dicots. 

Species

There are approximately 60,000 species of monocotyledons. The orchids (family Orchidaceae) are the largest family in this group (and in flowering plants in general) by the number of species, with over 20,000 species. True grasses (Poaceae), the most economically important monocotyledon family, contain about half as many species. Sedges, which are often mistaken for grasses, are monocots.

Monocotyledons produce the majority of the biomass produced in agriculture. These include forage grasses, sugar cane, and bamboos, as well as major grains (rice, wheat, maize, and so on). 

Other economically important monocotyledon crops include various palms (Arecaceae), bananas and plantains (Musaceae), gingers and relatives, turmeric and cardamom (Zingiberaceae), asparagus (Asparagaceae), pineapple (Bromeliaceae), sedges (Cyperaceae) and rushes (Juncaceae), and leeks, onion, and garlic (Leeks, Onion, and Garlic) (Amaryllidaceae). 

Monocotyledon epiphytes make up a large percentage of houseplants. Monocotyledons make up the majority of horticultural bulbs, which include lilies, daffodils, irises, amaryllis, cannas, bluebells, and tulips.

Evolution:

Monocots belong to a monophyletic group, which means they have shared evolutionary history. The monocots are thought to have descended from primitive eudicots. Given that the various physical characteristics of monocots are considered derived characteristics within the angiosperms, any plant that is more primitive than monocots in these several respects is a eudicot. 

Pollen grains from the Aptian Age of the Early Cretaceous Epoch (125 million to 113 million years ago) are among the earliest known monocot fossils. Monocots may have evolved as early as 140 million years ago, according to molecular clock studies (which use differences in DNA to estimate when a group split from its ancestors).

Physical characteristics

• Seeds with a single cotyledon, parallel-veined leaves, scattered vascular bundles in the stem, the absence of a typical cambium, and an adventitious root system are all characteristics of monocot plants. Pollen grains typically have a single aperture, and flower parts are typically in multiples of three (or furrow).
• A monocot’s roots lack a vascular cambium (the area where secondary xylem and phloem, or secondary vascular tissue, develop) and have no way of thickening secondary tissue. Monocot roots are structurally similar to eudicot roots in almost every way. Many eudicots have a taproot or several strong roots, as well as several orders of branch roots that all stem from the embryonic root (radicle). In such a system, the taproot or primary roots have a vascular cambium that is thickened by secondary growth. Monocots do not have this type of root system. Instead, the primary root that emerges from the embryo’s radicle aborts or remains undeveloped, resulting in the absence of a primary root.
• Monocot flowers differ from eudicot flowers primarily in the number of parts of each kind. The parts of monocot flowers are usually found in groups of three, occasionally four, but almost never five. The sepals and petals are particularly notable for their numbers. Even when the perianth is trimerous (in three sets), the stamens and pistils may be numerous, and the single ovary may have only two carpels instead of three. Six stamens, representing two whorls of three, are common.
• Pollen from monocots has one furrow or pore, while pollen from dicots has three.
• The embryo of a monocot has one cotyledon, whereas the embryo of a dicot has two.
• The vascular bundles in the stem of monocots are scattered, whereas in dicots they are arranged in a ring.
• Roots are adventitious in monocots (developing on a part other than the radical, such as stems and leaves), but they develop from the radicle in dicots (primary root and its lateral roots).
• The major leaf veins in monocots are parallel, whereas in dicots they are reticulated.

Cotyledons are an important component of a seed’s embryo. When a seedling germinates, they are the first parts of the plant to emerge from the soil. Plants with only one cotyledon are known as monocotyledons. They also have other features, such as flower parts that are in multiples of three. This means that their flowers could have three, six, or nine petals, or they could have sepals. They have ad hoc origins as well. Roots can emerge from almost any part of the plant that comes into contact with the soil, including the stem. A monocotyledon plant’s leaves also have parallel veins, which can be seen if you look closely at them.

Conclusion

A number of fundamental characteristics of monocotyledons, most notably the lack of a typical vascular cambium and parallel-veined rather than net-veined leaves, appear to have hampered evolutionary diversification. Within these constraints, monocots exhibit a wide range of structure and habitat diversity. On land, they have a cosmopolitan distribution. They can also be found in lakes, ponds, and rivers, where they can be found floating or rooted to the bottom. Some are submerged marine plants rooted to the bottom in fairly shallow water along the shore, while others grow in the intertidal zone along the seashore.