The essential component of sexual reproduction in plants is the seed. It is the end product of sexual reproduction and is only found in angiosperms or gymnosperms. Gymnosperms do not produce fruits or flowers, so their seeds are “naked”. Angiosperms on the other hand, have mature ovules which develop within the fruit after fertilization.
Angiosperms can be classified as monocots, or dicots, depending on how many cotyledons are present in the seed. The seed is composed of an embryo and a protective outer cover called the seed coating. There have been cases where some seeds were found to contain triploid endosperm. An embryo is made up of three parts: a radicle and an embryo axis.
Angiosperm seeds can be classified into two types based on how many cotyledons they contain.
- Monocotyledonous plants
- Dicotyledonous seed
What are Monocot Seeds?
Monocots will only have one seed leaf within their seed coat. It is usually a thin one because the endosperm that will feed the new plant isn’t within the seed leaf.
Monocot seeds are trimerous, meaning they have three parts. However, monocot seeds are more symmetrical than dicot seeds because there is only one cotyledon. Monocot seeds can be triangular, elliptic or ovate. After fertilization, ovules turn into seeds. The shape of the ovule will determine the final shape of the seed.
Monocot seeds are usually larger because of the large amount of endosperm. Endosperm is a storehouse of large amounts of food that provides nourishment for the embryo. Monocot seeds can also be called albuminous because they contain endosperm.
The embryo is the most vital part of a seed. It is covered with a cover and provided with nutrients and food by the endosperm. Monocots can be considered monophyletic because the evolutionary history of monocots can be traced back only to one ancestor.
Monocot seeds are: Rice, wheat, maize and bamboo are all examples.
Characteristics of Monocot Seeds
- Seed: Monocots only have one seed leaf within their seed coat. Because the endosperm that will nourish the new plant is not within the seed leaf, it is often a thin one.
- Germination: A monocot seed produces one leaf when it germinates. It is typically long and narrow like an adult leaf. It can be quite round, but there is only one seed in a monocot.
- Leaves: Monocot leaves are usually long and narrow with straight veins running up and down the leaf. Sometimes the veins run parallel from one edge to the other, sometimes from the centre of a leaf.
- Monocot Stems and Roots: Monocot stems are often unbranched and fleshy. They don’t get thicker each year. The older leaf will wrap the new leaves in a protective sheath. Dicot roots are often short and stringy. Many dicots have bulbs.
- Flowers: The monocot flower’s parts are in threes. It is possible to see six petals in a flower by having the sepals the same colour as its petals. Usually, there are the same number as petals of stamens.
- Monocot Seedpods: They are usually made up of three parts. The seeds are usually large and fleshy. Monocot seeds include the Coco-de-Mer, the largest seed in the universe, and Orchid seeds which are the smallest.
Structure Monocotyledonous Seed
Seed coat
The seed coat, which is the outermost layer of the seed, may remain bonded to the fruit wall in some cases. Two layers of cells, called integuments, make up the seed coat. The tissue comes from the mother plant, where the inner layer is the tegmen and outer the testa.
Some monocots have layers of seed coat that aren’t distinct. They might even be fused to the fruit wall in some cases. If the outer layer is distinct, it may contain patterns or patches of hair. The ovule characteristic determines the number of layers in the seed coat. The inner layer of bitegmic Ovules can be left as one layer, or it can split to form two or more layers that accumulate food material.
The outer layer also contains cells that have tannin deposits. This gives the seed coat a dark appearance. The outer layer continues to deposit different substances as the cells in the seed coat grow larger. This causes their walls to thicken.
The hilum, an oval depression in the seed coat that represents the location where the ovules were attached at the wall of the egg, is known as the seed coat. Some seeds’ seed coats may have wings or hair that aid in wind dispersal. Some seed coats also have waterproof materials that protect them from drying out or decaying when water is used to disperse the seeds.
Cotyledon
The embryonic leaf of the seed, the cotyledon, occupies the largest space and provides nutrients and protection for the embryo. Dicotyledonous seeds have two cotyledons, whereas monocotyledonous seeds only have one. A seed’s number of cotyledons is one way to distinguish flowering plants from other groups.
Monocot plants have a modified cotyledon that forms a scutellum. This is non-photosynthetic, and absorbs stored food from the endosperm. There are 24 cotyledons in cypress and pines, which have more than 2 cotyledons. Different species have different cotyledon life spans. Some last for only a few days while others can last up to a whole year.
Hypocotyl
Hypocotyls are the area of the seed below the seed leaves (or the cotyledons), and bove the Radicle. Although hypocotyl eventually forms part of the stem it serves an essential function in pushing the cotyledons out of the soil during germination. Although hypocotyl does not grow as fast as epicotyl it is still the first structure that emerges from the soil. The radical and the hypocotyl make way for the epicotyl. However, epicotyl cells are fragile and could be damaged during growth.
Embryo
The embryo is a simple multicellular structure made up of undifferentiated cells that has been created by fertilization of haploid eggs cells by sperm cells. The embryo is a part of a seed. It is protected by various structures, such as endosperm or seed coat. After fertilization, the zygote undergoes asymmetrical cell division. An embryo formed by an asymmetrical division will have a small and large cell. The embryo is made up of DNA from both the ovule and pollen that forms a zygote.
The connective is formed by the smaller apical cells. Connective is what connects the embryo and the endospore. It allows the embryo to continue to grow and separate into different regions where cell division takes place and other non-reproductive activities such as metabolism, respiration, storage.
Monocots have an embryo in a groove that runs along the endosperm. It is surrounded by a larger, shield-shaped cotyledon known as scutellum. Other structures such as plumule and radicle, hypocotyl and epicotyl are found in the embryo axis. These structures eventually lead to embryogenesis, which is when a new plant forms. The embryo contains the most lipid-soluble vitamins and lipids of all parts of the seed.
Epicotyl
The epicotyl region is located above the stalks on the seed leaves and is crucial for plant germination. The epicotyl region controls the growth of the stem, which extends above the soil surface.
Hypogeal germination is observed in epicotyl, which grows faster than the other parts of an embryo. The growth of plumule above soil, while the cotyledons remain below the soil is hypogeal germination. The epicotyl region is the attachment point between the shoot apex (the embryo’s first true leaves) and the growth of cells in the epicotyl. Monocots and dicots have different definitions of epicotyl. Monocots refer to the shoot that grows from the soil or seed as the epicotyl, while dicots refer to the area of the shoot located above the cotyledons.
Radicle
Radicle is the embryo’s region. It is the area that emerges from the seed during germination and eventually leads to the formation roots. Radicle, also known as the embryonic root, is positively geotropic and roots downwards in the soil. Through the micropyle, the radicle emerges from the seed.
There are two types of radicels. Antitropous radicles face away from the seed coat’s hilum, whereas syntropous point towards it. In monocots, the radicle is protected by a thin sheath called coleorhiza. It is not in dicots. However, the root cap protects the radicle during its early stages of growth.
Plumule
The plumule is part of the seed embryo and eventually becomes the shoot with vegetative parts such as leaves and stems. Plumule, which is a negatively geotropic radical unlike radical, might contain leaf structure in the seed. It is commonly referred to as the embryonic shot and appears as a bud on one of the axise of the embryo.
Some plants, such as sunflower, have no leaf structure. The growth doesn’t happen until the cotyledons above the ground. However, in some cases, there is a leafy structure that develops from the soil, with the cotyledons remaining below the surface. Monocots have a plumule that is surrounded and protected by a coleoptile, which is absent from the plumules in dicot seeds.
Endosperm
Endosperm refers to the mass of tissues that are formed in the embryo during fertilization. Endosperm serves two primary purposes: to provide nutrition and to protect the embryo. Endosperm cells are unique because they are triploid, with three sets of DNA per nucleus.
Endospore is formed when one of the sperm cell’s diploid central cells fertilizes with the female gametophyte’s diploid central cell. This creates a primary endosperm cell that has a triple fusion nucleus. The majority of flowering plants are polyploidy. However, some may have a triploid or diplod set of chromosomes.
Three types of cells make up the endosperm: the starchy endosperm cell cells, the basal layer and the aleurone. Monocots have a large endosperm, as the embryo’s primary source of nutrition. The two cotyledons provide the nutrients in dicots.
The starchy endosperm occupies most of the endosperm. Endosperm is made up of dead cells that are filled with protein bodies and starch granules. Basal cells are characterised by cell wall ingrowths that have a cell membrane 22 times thicker than a normal plant cell. Although the aleurone layer is one-cell thick, it can also be three-layered in monocots. This layer surrounds the embryo and starchy endosperm.
Features used to Distinguish Monocots from Dicots
Monocots are distinguished from dicots by six distinct structural characteristics. The mature angiosperm has five of these characteristics: the flowers and leaves, roots, stems and pollen grains. The seed is what creates the greatest difference between monocots or dicots.
Flowers
The parts of flowers are usually arranged in circles. The reproductive parts are in the middle, surrounded by sepals and petals. These flower parts are trierous in monocots. Monocots have flower parts that are either trimerous or structured in multiples of three. This means that the monocot’s flower parts are usually arranged, numbered, or structured in multiples of three.
Stems
Monocot stems are no longer able to grow larger by secondary growth, producing wood and bark. Monocot stems are allowed to die each year and allow new stems growth. Monocot stems are incapable of growing side stems or branches. The top is the only place that can grow. Monocotyledons tend to be small and herbaceous.
An epidermis and hypodermis are found in a monocot stem’s cross section. Also, ground tissues and vascular bundles can be seen. Monocot stems typically have the following characteristics: a single layer epidermis, thick cuticle, lack of epidermal hairs, lack of concentric arrangement, hypodermis that is sclerenchymatous, presence of bundle sheaths, oval vascular bundles with different sizes, and, most importantly, scattered vascular bundles which do not create any pattern.
Leaf Veination
Veination is the arrangement of veins within a leaf blade. These veins transport water and carbohydrates throughout plants. These veins are parallel-like in monocots. Parallel veination is smaller than other types of veination. They are also smaller with smaller veins connecting them.
Embryos
The plant embryo is the portion of the seed that contains all the precursor tissues and one or more of the cotyledon. Monocots, as the name implies, have one (mono-), cotyledon and one leaf emerging out of the cotyledon. Monocots’ seed pods are also trimerous, in parts of three, because their carpel grew from three parts.
The cotyledon, which is the first part of a plant to emerge from a seed, is what is used to distinguish the main angiosperm groups. Cotyledons play an important role in food absorption. They absorb nutrients from the surrounding environment until the plant can photosynthesise its own nutrients.
Roots
Monocots begin with tap roots, but these tap roots die quickly after germination, and are replaced by adventitious root. Adventitious roots are fibrous and spread throughout the soil in many directions. They prefer to be in the soil’s upper layer and can be modified for other purposes such as additional anchorage or aerial support. We can grow multiple plants from stems or leaf cuttings from a pre-existing plant because adventitious roots are often formed from an organ other than the root.
Pollen Grains
Monocots retain the pollen structure from their first angiosperms. Monocot pollen grains are monosulcate. This means that there is only one furrow or pore in the outer layer of the pollen.
Examples of Monocots
Bananas
The banana plant, often mistakenly thought to be a tree is actually a monocot. It is closely related with the grass family. Like monocots, banana plants don’t have secondary growth. They die down after producing their fruits. Even banana fruits can grow in three parts (tri-locular with three segments), and they have familiar parallel veins on their leaves.
Lillies
The lily is probably the easiest monocot to identify because it has all the characteristics of monocots. Most lilies have obvious flowers that are trimerous. The three pedals of most lilies are identical in size and shape. The roots are adventitious and small.
Some lilies, however, have different flowering and petal structures which can make it difficult to classify them as monocots or dimocots. There are many shapes for the flowers, including trumpets, funnels and cups, bells and flat ones. A spadix is a stem that holds the flowers of peace lilies. It doesn’t look like a typical flower cluster. One common misconception about peace lily flowers is that they only have one petal. The petal that most people mistakenly believe is the spathe is actually a special leaf. True flowers are trimerous on the spadix.
Orchids
The primary characteristic that makes orchids monocots, similar to the lily is their flower. Although the petals of orchid flowers are clearly trierous, there are some morphological characteristics that distinguish them from other monocots. One petal of the many petals that are arranged in groups of three has evolved into a lip. This is a special landing platform for pollinators. The three petals of an orchid are not identical, so it can be difficult to identify monocots. The orchid’s stigmatic lobes are three as is the norm with monocots. However, they are fused later with only a few lines to indicate its trimerous structure.
Grasses
While grass is not usually thought of as a flowering plant but they do have tiny flowers at their tips! The grass family is the most important monocot group. Consider corn, wheat, rice, and other monocots. They all have flowers that aren’t adorned with petals or sepals, but they are all examples of grasses.
Palm Trees
Monocot plants are not common, with the exception of the palm tree. Monocots are unable to grow as tall and large as palm trees because they lack secondary development, which is the growth of wood and bark. This limits monocots’ ability to be herbaceous. Palm trees have overcome this problem by using their vascular bundles as well as the lignin in them to make a firmer stem. Parenchymal cells, which surround the vascular bundles and thicken palm stems, provide additional support to tall tree-forms.
One characteristic that distinguishes a palm tree as monocot is its leaf. Palm tree leaves are strap-like and long, with major veins running parallel.
Monocots can also be found in white trillium, Dwarf Daylily and Tulips.
Other Examples of Monocot: White Trillium, Dwarf Daylily, Tulips, Snowdrops, Crocus, Daffodil, Aloe vera, Knights Lily, Iris, Spiderworts, etc
References
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