MONOCOTYLEDONOUS AND DICOTYLEDONOUS SEEDS: CHARACTERISTICS, EXAMPLES - BIOLOGY - 2024

The monocots and dicots seeds differ primarily in the number of cotyledons or primary leaves. In monocots the seeds contain a single cotyledon, in dicots there are two cotyledons.

Cotyledons differ from the rest of the secondary leaves of the plant by shape and size. Likewise, they constitute a storage organ for the new plant, since they contain nutritional elements such as starch, glucose, proteins, minerals and fats.

Variety of seeds. Source: pixabay.com

Because cotyledons store reserve substances to meet the requirements of the seedling during germination, they tend to have a fleshy appearance. These structures are attached to the embryonic axis through the node, and open like a notebook.

The apical end of the axis in relation to the point of attachment of the cotyledons is called the epicotyl or first internode of the plant. As for the portion that is downwards, it is called hypocotyl, and represents the shoot that will become the root.

Seed characteristics

The seed contains the embryo of the new plant in a state of dormancy or latent life. This embryo is the result of the fertilization process of the egg cell; in angiosperms, double fertilization gives rise to the embryo and the endosperm.

The embryo is made up of the radicle, which will give rise to the primary root through the micropile. Likewise by the hypocotyl or stem axis, which in epigeal germination raises the cotyledons above the soil surface.

On the other hand, the cotyledons will be the first leaves and serve to absorb the nutrients stored in the endosperm of the seed. In addition to the gémula or plumule that corresponds to the caulinar apex, and some foliar primordia.

In seedless or pteridophytic plants, the embryo is of the unipolar type, there is a growth axis from the stem with numerous adventitious roots. On the contrary, in spermatophytes or phanerogams - plants with seeds - the embryo is of the bipolar type, in one axis the stem is formed and in the other the root.

In gymnosperms the embryo is made up of several cotyledons, two in the Ginkgoaceae and more than five in the Pinaceae. Dicotyledons have two cotyledons of different shapes and sizes - fleshy, foliaceous, curly, folded - depending on each species, genus and family.

In monocots the cotyledon is unique, it is located laterally similar to the plumule. As for grasses, the embryo presents a high degree of development, dividing into duly differentiated parts.

A developed grass embryo has the scutellum, plumule, coleoptile, coleorhiza, root primordium, and epiblast. There are special cases, such as the Orchidaceae, which present an undifferentiated embryo lacking cotyledons and radicle, only presenting plumule.

Differences

Monocotyledons

Monocots contain a single cotyledon within the seed coat. It is usually a thin leaf since the endosperm necessary to feed the new plant is not found within the cotyledon.

During the germination process of a monocot, a single leaf is produced. This first embryonic leaf is usually long and narrow -Family Iridaceae-, in some species it can be rounded -family Liliaceae-.

Corn seeds (Zea mays). Source: jacilluch

Germination begins when the seeds absorb water to soften the testa and initiate biochemical activities. Monocotyledonous seeds store a higher starch content, which is why they require approximately 30% humidity to germinate.

In monocots, the emerging radicle is covered by a protective sheath or coleorhiza. In addition, the leaves that emerge from the seedling are covered by a layer called the coleoptile.

Dicotyledons

Dicots contain two cotyledons within the seed coat. They are generally round and thick, as they contain the endosperm necessary to feed the embryonic plant.

In the germination of a dicotyledonous seed, two leaves are produced that contain the nutritional reserves for the new plant. These leaves are generally thicker and remain on the plant until the true leaves develop.

Sunflower seeds (Helianthus annuus). Source: pixabay.com

Dicotyledonous seeds have a higher content of fats and oils as storage and reserve substances. For this reason the seed needs to reach at least 50% humidity to start the germination process.

In dicotyledons, the radicle or primary root emerges from the seed, favoring the absorption of moisture for the new plant. The apical meristem eventually develops from the radicle giving rise to the root system, later the cotyledons, hypocotyl and epicotyl emerge.

Germination

The conditions for the germination process of monocotyledonous and dicotyledonous seeds to occur are similar. Both types of seeds must be fully developed, with a viable embryo, wet endosperm, appropriate number of cotyledons, and stable coat or testa.

The endosperm and the cotyledons are responsible for supporting the growth of the seedling, providing food until photosynthesis begins. Germination requires favorable environmental conditions, especially temperature, light and humidity.

Epigeal germination. Source: pixabay.com

The temperature must be warm to promote cellular respiration, but not so high that it can damage the seed, nor so low that it causes dormancy. Likewise, humidity, solar radiation, the presence of oxygen and carbon dioxide contribute to the germination of the seed.

For dicots, depending on the species, there are two types of germination: epigeal and hypogeal. In epigeal germination the cotyledons emerge from the soil as a consequence of the growth of the hypocotyl.

In hypogeal germination, the cotyledons remain underground, only the plumule emerges to the surface. The cotyledons eventually decompose, as the plant continues to grow and the first photosynthesizing organs of the plant appear.

In both monocots and dicots, seedlings develop slowly after emerging on the soil surface. The seedling initially develops the roots and later the true leaves necessary to initiate photosynthesis and convert light into energy.

Examples

Monocot seeds

Rice grains (Oryza sativa). Source: pixabay.com

• Rice (Oryza sativa)
• Barley (Hordeum vulgare)
• Millet (Eleusine coracana)
• Corn (Zea mays)
• Sorghum (Sorghum bicolor)
• Baker wheat (Triticum aestivum)

Dicotyledonous seeds

Carica papaya seeds. Source: pixabay.com

• Pea (Pisum sativum)
• Sunflower (Helianthus annuus)
• Mahua or butter tree (Madhuca longifolia)
• Papaya or milky (Carica papaya)
• Radish (Raphanus sativus)
• Castor or castor (Ricinus communis)

References

1. Dotson J. Dianne (2018) Sequence of Steps in Monocot & Dicot Germination. Sciencing. Recovered at: sciencing.com
2. Seed Germination (2003) Polytechnic University of Valencia. Part III. Topic 17. Recovered in: euita.upv.es
3. González Ana María (2016) Morphology of Vascular Plants. Item 6 Seeds. National University of the Northeast, Corrientes, Argentina. Recovered at: biologia.edu.ar
4. Megías Manuel, Molist Pilar & Pombal Manuel A. (2018) Seed. Atlas of Plant and Animal Histology. Department of Functional Biology and Health Sciences. Faculty of Biology. University of Vigo. 9 pp.
5. Monocots and Dicots. Chart showing Differences (2018) The Seed Site. Recovered at: theseedsite.co.uk
6. Monocots versus Dicots. The Two Classes of Flowering Plants (2018) UCPM University of California Berkely. Recovered at: ucmp.berkeley.edu