Monocots and dicots posses obvious physical differences and are easy to identify, with few exceptions. Differences are observed in the appearance of the embryo, presence of furrows or pores, number of petals, and major leaf veins. Other differences include the stem vacuoles, roots, and secondary growth.
While there are several differences between monocots and dicots, they are named for the number of cotyledons that sprout from the seed until the seedling produces real leaves. Cotyledons look like little leaves. Monocots have one "leaf," and dicots have two "leaves."
Flowering plants are in the Phylum, or Division, Anthophyta in the Kingdom Plantae. Anthophyta are the flowering plants. It is further divided into two classes: monocotyledons and dicotyledons (also monocotyledones and dicotyledones and often referred to as monocots and dicots). The monocot class is also called Liliopsida, and the dicot class is Magnoliopsida.
The Cotyledon
According to the December 1999 article, "Aperture," by Owen Davis, Professor of Geosciences at the University of Arizona, "Apertures are any thinning, thickening or other modification of the wall of pollen or spores that serve as an exit for its contents or to allow shrinking and swelling of the grain in response to changes in moisture content." Pores and furrows are apertures.
Monocots are characterized by one furrow or pore. Most dicots have three furrows or pores.
Number of Petals and Differences in Leaf Structure
The number of petals is a fairly reliable indicator between monocots and dicots. Monocots usually have three petals or a number of petals in multiples of three, such as six or nine. Dicots have petals in multiples of four or five.
Monocot leaves usually have major leaf veins that run parallel. All of the veins run the length of the leaf, as in a blade of grass. Some monocots do have leaves like dicots, however. Dicot leaves have major leaf veins that branch out, like the branches on a tree. They also have many smaller veins that form a network, like a web. A maple tree leaf is an example of a dicot leaf.
Vascular Bundles in the Plant Stem Structure
Vascular bundles transport food, organic materials, and minerals throughout the plant. They run the length of the stem, but the location in the plant stem structure is different between monocots and dicots. Dicots have vascular bundles that are arranged in a ring. For example, if one cuts a tree and looks at the cross section on the top of the stump, the vascular bundles would appear as a ring. Monocot vascular bundles are scattered throughout the stem.
According to Robert J. Brooker, et al., "In the primary stems of beans and other eudicots [dicots], the vascular bundles are arranged in a ring, which is easily seen in thin slices made across a stem. Celery strings are examples of eudicot vascular bundles. In contrast, vascular bundles are scattered in the stems of corn and other monocots."
Roots and Plant Stem Structural Support (Secondary Growth)
Both monocots and dicots initially produce a primary root. The monocot root dies off and smaller, fibrous roots develop from the stem. Brooker, et al., explain that the dicot root continues to produce root tissue and branches to produce a taproot system.
In the University of California Museum of Paleontology article, "Monocots versus Dicots: The Two Classes of Flowering Plants," Brian Speer says, "Most seed plants increase their diameter through secondary growth, producing wood and bark." Most dicots produce this secondary growth. All monocots and some dicots do not, but they are supported by other means. Speer explains that, "Palms rely on overlapping leaf bases, thickened enlarged cells, and prop roots to stay up."
It is important to remember that the Division Anthophyta is further divided into monocotyledons and dicotyledons. Anthophyta are flowering plants, including trees such as maples and cherry trees. Speer points out that pine trees are conifers that do not produce flowers, so they do not belong to the Division Anthophyta, but to Coniferophyta, which has different classes.
The characteristic that truly distinguishes monocots from dicots are the number of leaf-life cotyledons – one or two – that sprout from a seed. Other characteristics are used to identify whether a plant is a monocot or dicot, but there are exceptions to these guidelines.
Reference
Brooker, Robert J., Widmaier, Eric P., Graham, Linda E., and Stiling, Peter D. Biology. New York: McGraw-Hill, 2008.
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