Dicotyledon
Dicotyledons, commonly referred to as dicots, are a major group of flowering plants characterized by having two embryonic leaves, known as cotyledons, in their seeds. This group is one of the two primary categories of angiosperms, the other being monocotyledons, which contain only one cotyledon. Dicots encompass over 175,000 species, including important agricultural crops such as tomatoes, beans, and potatoes, as well as various tree species like oaks and walnuts.
In addition to their two cotyledons, dicots typically feature broad leaves with a network of branching veins, a single large taproot system, and flowers that usually have parts in multiples of four or five. Their vascular tissues are arranged in concentric rings, which distinguishes them from monocots, which have scattered vascular structures. Interestingly, modern research indicates that dicots represent a polyphyletic group, meaning that their defining characteristics evolved from multiple ancestors rather than a single common ancestor. This complexity has prompted ongoing efforts to refine plant classification systems, notably through the work of the Angiosperm Phylogeny Group. Overall, understanding dicots is crucial for both ecological studies and agricultural practices due to their significant role in human sustenance and the environment.
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Dicotyledon
A dicotyledon (pronounced dye-kott-uh-LEE-don) is any member of a group of flowering plants that has a pair of embryonic leaves called cotyledons inside its seeds. Dicotyledons (commonly called "dicots") are one of two traditional categories of flowering plants; plants in the other category are called monocotyledons and have only a single cotyledon within their seeds. More than 175,000 species of plants belong to the dicot family. These include many species that are of vital importance to humankind, including tomatoes, peas, beans, potatoes, and oranges and trees such as oaks and walnuts.
Background
All living organisms are split into various categories based upon shared characteristics. These shared traits reflect biological qualities that indicate how these organisms diverged from one another during the evolutionary process. For instance, one such evolutionary adaption among plants was the development of a reproductive process that helped them adapt to conditions on land. As part of this process, an embryo (called a sporophyte) is enclosed within a special type of tissue (called gametophytic tissue) such as a seed or the archegonia of mosses.
Green plants with this adaptation are collectively called embryophytes. The vast majority of embryophytes may be split into two categories: angiosperms and gymnosperms. Other smaller clades of green plants, including mosses, ferns, and liverworts, exist, but most land plants are placed into the angiosperm or gymnosperm groups. Gymnosperms are plants that have unprotected seeds. The most familiar types of gymnosperms are pine trees, cycads, and ginkgo.
Angiosperms, which include all flowering plants, are believed to have evolved from an early form of gymnosperm. Angiosperms are plants whose seeds are enclosed, such as within fruit. Angiosperms also have other shared characteristics, such as the development of flowers, the presence of a reproductive stamen that produces pollen, and the production of endosperm in seeds. Endosperm is a type of tissue that feeds the developing embryo and its cotyledons. Angiosperms make up about 80 percent of all known living plants.
Angiosperms are divided into dicots and monocots. Angiosperms as a class are monophyletic, which means that they arose from a single shared ancestor. Organisms defined as angiosperms derive their unique traits from the same single species that ultimately evolved into a multitude of dicots and monocot species. Like angiosperms as a whole, monocots are monophyletic—they had one shared ancestor from which their shared characteristics originate.
However, molecular analyses of dicots demonstrate that the characteristics used to distinguish dicots from monocots have a polyphyletic source. This means that while all monocots and dicots have a shared ancestor, the more recent evolutionary characteristics that differentiate dicots from monocots did not come from one ancestor. Instead, dicots derive their distinctive traits from multiple ancestors of different origin, which evolved both before and after the divergence of monocots. Dicots independently developed similar traits through environmental pressures rather than as a result of genes having been handed down from one shared species, as with the monocots.
Overview
Botanists have sought to categorize flowering plants since Theophrastus first created a system of categorization for plants in the fourth century BCE. However, Theophrastus believed that the underlying differences among various plants could be seen in traits such as whether a plant grew as a vine, a tree, or an herb and whether its life span could be described as annual, biennial, or perennial.
In modern science, the cotyledon is the primary identifying trait used to classify flowering plants as monocots or dicots. Cotyledons serve an important function in all flowering plants. These leaves form inside the seed after being germinated. In dicots, cotyledons use photosynthesis to feed the young plant shortly after emerging from the seed.
In addition to a pair of cotyledons, dicots share several other characteristics. One is leaf shape. This aspect was one of the principal features used by British naturalist John Ray to classify flowering plants as monocots and dicots in his Methodus Plantarum Nova (New Plant Method) in 1682. Dicot leaves tend to be more rounded than monocot leaves, which are typically long and narrow. Also, the veins of dicot leaves form a branched pattern that resembles a net, whereas the veins of monocot leaves form a parallel pattern.
The roots of dicots and monocots also differ. Mature monocots form a network of branching fibrous roots that absorb nutrients from the soil and grip the plant firmly into the soil. Dicots, however, rely upon a single large root called a taproot. In dicots, smaller roots branch off from the taproot in horizontal directions.
Dicots and monocots have different pollen and flower anatomy. In monocots, each speck of pollen typically has a single furrow or pore on its surface. In dicot plants, the pollen is more likely to have three furrows on its surface. Flowers are composed of sepals, petals, and stamens. In monocots, these flower parts are generally found in multiples of three, while in dicots they tend to grow in multiples of four or five. In monocots, the vascular tissues of the plant (which transport water and nutrients) are scattered unevenly throughout the stem. In dicots, the vascular system is arranged in rings.
The simple separation of angiosperms into either monocots or dicots has long been perceived as potentially problematic. When systematic analyses of dicots confirmed that they were a polyphyletic set of organisms without a single shared ancestor separate from monocots, grouping dicots together as a single clade was deemed questionable. In fact, on an evolutionary scale, some groups of dicots were found to be more closely related to monocots than to other dicots. Certain plants seemed to share characteristics that are more typically assigned to either monocots or dicots. For instance, water lilies of the Nymphaeales family share a vascular system that more closely resembles dicots but a pattern of veins in its leaves like that of monocots. They also have an unusual cotyledon. Botanists are uncertain whether the cotyledon in embryonic water lilies is composed of a single leaf (monocot) with two lobes or a pair of leaves (dicot) that are fused together.
Scientists have sought to reorganize the classification system of plants to resolve these issues. As part of these efforts, several new orders of families of flowering plants have been established by the Angiosperm Phylogeny Group (APG), a group of international botanists seeking to create a consensus regarding the taxonomic status of flowering plants. The group first proposed a new classification system called the APG I system in 1998. It has undergone several revisions since its initial release. A new version called the APG IV system was issued in 2016.
Bibliography
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