Dicotyledons


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Dicotyledons

A large group of flowering plants (angiosperms) that for many years has been considered one of the two main categories of plants, the other being monocotyledons. Dicotyledons have two seedling leaves as opposed to the single one in most monocotyledons. Several deoxyribonucleic acid (DNA) sequence studies subsequently demonstrated that there are two groups of angiosperms, but these correspond not to the number of seed leaves but to the two major pollen types. Thus, the term “dicotyledon” is no longer meaningful because some plants of this type are more closely related to monocotyledons. The group of former dicotyledons, which have pollen with a single aperture, includes magnolia, avocado, black pepper, and pipeworts; they are now termed magnoliids and include monocotyledons. The other category of dicotyledons, those with three (and often more) apertures in their pollen, are called eudicotyledons (true dicotyledons). See Eudicotyledons, Flower, Magnoliophyta, Monocotyledons, Plant kingdom

Dicotyledons

 

dicots, dicotyls (Dicotyledoneae. Dicotyledones, or Magnoliatae), a class of angiospermous plants, characterized by the presence in the embryo of two lateral, opposite cotyledons (hence the name). In dicotyledons, in contrast to monocotyledons, the vascular conveying bundles are located in a ring formation, and between the xylem and the phloem is a special formative tissue —cambium—that provides for secondary growth; the leaves, as a rule, are net-veined; the number of flower parts (sepals, stamens, and carpels) is usually a multiple of four or five. The rootlet of the embryo most often becomes the principal root, capable of many years of existence: the leaf blade is often segmented and its edges notched or serrate. Some representatives of dicotyledons have atypical characteristics, and sometimes they have a few isolated characteristics that are more typical of monocotyledons. Dicotyledons are the broadest group of flowering plants, with approximately 175,000 species (350–360 families), which exceeds the number of monocotyledons by about three times. Of flora in the USSR, more than 15,000 species of 1,370 genera and approximately 125 families belong to the dicotyledons. Dicotyledons are characterized by diversity of vegetative and reproductive organs, which greatly hampers elucidation of the true relationships between orders and families. The ancestors of the dicotyledons, as well the time and place of their origin, are not yet clear. The most widely accepted hypothesis is that the so-called polycarpous plants (the Magnolia-les and Ranales orders and others) is the oldest parent group in the evolution of angiosperms.

Dicotyledons occupy a most important place in man’s economic activity. To them belong food and fodder plants (potato, buckwheat, soya, sugar beet, melons, and many others); fruit and berry crops (grapes, citrus fruit, apple, currant, and others); oil-producing plants (sunflower, peanut, and tung tree); the majority of tree species (oak, birch, linden, and others); tea, coffee, cacao, and hundreds of the most important medicinal plants, spices, and aromatic plants (laurel, cinnamon, and others); tobacco; the most important fibrous plants (cotton, flax, hemp, jute, and others): plants that yield rubber, gums, and resins; and many tanning, dyeing, volatile-oil, and ornamental plants.

REFERENCES

Flora SSSR. vols. 5–30. Moscow-Leningrad, 1936–60.
Kozo-Polianskii, B. M. Kurs sistematiki vysshikh rastenii. Voronezh, 1965.
Takhtadzhian, A. L. Sistema ifilogeniia tsvetkovykh rastenii. Moscow-Leningrad, 1966.
Engler, A. Syllabus der Pflanzenfamilien, 12th ed., vol. 2. Berlin, 1964.
Cronquist, A. The Evolution and Classification of Flowering Plants. London-Edinburgh, 1968.

M. E. KIRPICHNIKOV

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Hickey LJ (1973) Classification of the architecture of Dicotyledons leaves.
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Tension wood is formed by woody dicotyledons to generate the force necessary for reorientation of branches and stems following gravistimulation or loss of apical dominance.
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