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A cotyledon (ËŒkoÊŠtÉ›lÉªdÉ”Ë�n; "seed leaf" from Greek: ÎºÎ¿Ï„Ï…Î»Î·Î´ÏŽÎ½ kotylÄ“dÅ�n, gen.: ÎºÎ¿Ï„Ï…Î»Î·Î´ÏŒÎ½Î¿Ï‚ kotylÄ“donos, from ÎºÎ¿Ï„Ï�Î»Î· kotÃ½lÄ“ "cup, bowl"), is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants (angiosperms). Species with one cotyledon are called monocotyledonous (or, "monocots") and placed in the class Liliopsida. Plants with two embryonic leaves are termed dicotyledonous ("dicots") and placed in the class Magnoliopsida.
In the case of dicot seedlings whose cotyledons are photosynthetic, the cotyledons are functionally similar to leaves. However, true leaves and cotyledons are developmentally distinct. Cotyledons are formed during embryogenesis, along with the root and shoot meristems, and are therefore present in the seed prior to germination. True leaves, however, are formed post-embryonically (i.e. after germination) from the shoot apical meristem, which is responsible for generating subsequent aerial portions of the plant.
The cotyledon of grasses and many other monocotyledons is a highly modified leaf composed of a scutellumand acoleoptile. The scutellum is a tissue within the seed that is specialized to absorb stored food from the adjacentendosperm. The coleoptile is a protective cap that covers the plumule (precursor to the stem and leaves of the plant).
Gymnosperm seedlings also have cotyledons, and these are often variable in number (multicotyledonous), with from 2 to 24 cotyledons forming a whorl at the top of the hypocotyl (the embryonic stem) surrounding the plumule. Within each species, there is often still some variation in cotyledon numbers, e.g. Monterey Pine (Pinus radiata) seedlings have 5â€“9, and Jeffrey Pine (Pinus jeffreyi) 7â€“13 (Mirov 1967), but other species are more fixed, with e.g. Mediterranean Cypress always having just two cotyledons. The highest number reported is for Big-cone Pinyon (Pinus maximartinezii), with 24 (Farjon & Styles 1997).
The cotyledons may be ephemeral, lasting only days after emergence, or persistent, enduring a year or more on the plant. The cotyledons contain (or in the case of gymnosperms and monocotyledons, have access to) the stored food reserves of the seed. As these reserves are used up, the cotyledons may turn green and begin photosynthesis, or may wither as the first true leaves take over food production for the seedling.
Epigeal versus hypogeal development
Cotyledons may be either epigeal, expanding on the germination of the seed, throwing off the seed shell, rising above the ground, and perhaps becoming photosynthetic; or hypogeal, not expanding, remaining below ground and not becoming photosynthetic. The latter is typically the case where the cotyledons act as a storage organ, as in many nuts and acorns.
Hypogeal plants have (on average) significantly larger seeds than epigeal ones. They also are capable of surviving if the seedling is clipped off, as meristem buds remain underground (with epigeal plants, the meristem is clipped off if the seedling is grazed). The tradeoff is whether the plant should produce a large number of small seeds, or a smaller number of seeds which are more likely to survive.
Related plants show a mixture of hypogeal and epigeal development, even within the same plant family. Groups which contain both hypogeal and epigeal species include, for example, the Araucariaceae family of Southern Hemisphere conifers, the Fabaceae (pea family), and the genus Lilium(seeLily seed germination types).
The term cotyledon was coined by Marcello Malpighi. John Ray was the first botanist to recognise that some plants have two and others only one, and eventually the first to recognise the immense importance of this fact to systematics.
The spermatophytes (from the Greek word "Î£Ï€ÎµÏ�Î¼Î±Ï„ÏŒÏ†Ï…Ï„Î±") (also known as phanerogams) comprise those plants that produce seeds. They are a subset of the embryophytes or land plants. The living spermatophytes form five groups:
- cycads, a subtropical and tropical group of plants with a large crown of compound leaves and a stout trunk,
- Ginkgo, a single living species oftree,
- conifers, cone-bearing trees and shrubs,
- gnetophytes, woody plants in the genera Gnetum,Welwitschia, andEphedra, and
- angiosperms, the flowering plants, a large group including many familiar plants in a wide variety of habitats.
In addition to the taxa listed above, the fossil record contains evidence of many extinct taxa of seed plants. The so-called "seed ferns" (Pteridospermae) were one of the earliest successful groups of land plants, and forests dominated by seed ferns were prevalent in the late Paleozoic. Glossopteriswas the most prominenttreegenus in the ancient southern supercontinent of Gondwana during the Permian period. By the Triassic period, seed ferns had declined in ecological importance, and representatives of modern gymnosperm groups were abundant and dominant through the end of the Cretaceous, when angiosperms radiated. Another Late Paleozoic group of probable spermatophytes were the gigantopterids.
Relationships and nomenclature
Seed-bearing plants were traditionally divided into angiosperms, or flowering plants, and gymnosperms, which includes the gnetophytes, cycads, ginkgo, and conifers. Older morphological studies believed in a close relationship between the gnetophytes and the angiosperms, in particular based on vessel elements. However, molecular studies (and some more recent morphological and fossil papers) have generally shown a clade of gymnosperms, with the gnetophytes in or near the conifers. For example, one common proposed set of relationships is known as the gne-pine hypothesis and looks like:
However, the relationships between these groups should not be considered settled.
- Division Spermatophyta
A more modern classification ranks these groups as separate divisions (sometimes under the Superdivision Spermatophyta):
- Cycadophyta, the cycads
- Ginkgophyta, the ginkgo
- Pinophyta, theconifers
- Gnetophyta, the gnetophytes
- Magnoliophyta, theflowering plants
From Yahoo Answers
Answers:Grass is the classic example of a monocotyledon plant. The first shoot sent up on germination is a single leaf. A pea is a good example of a dicotyledon. Cotyledon is the first leaf that appears on germination. 'Mono' means one, 'di' means two. The traditional differences between monocots and dicots are: Flowers: In monocots, flowers are trimerous (number of flower parts in a whorl in threes) while in dicots the flowers are tetramerous or pentamerous (flower parts are in fours or fives). Pollen: In monocots, pollen has one furrow or pore while dicots have three. Seeds: In monocots, the embryo has one cotyledon while the embryo of the dicot has two. Stems: In monocots, vascular bundles in the stem are scattered, in dicots arranged in a ring. Roots: In monocots, roots are adventitious, while in dicots they develop from the radicle. slice of onion, showing parallel veins in cross section slice of onion, showing parallel veins in cross section Leaves: In monocots, the major leaf veins are parallel, while in dicots they are reticulate. Not all of these, though, are necessarily definitive. The leaves of most pine trees (which are multicotyledinous) have parallel veins, for example. There is a good picture of a monocot and a dicot seedling side by side here: http://en.wikipedia.org/wiki/Cotyledon
Answers:There's a very easy way to tell whether a plant is dicot or monocot. Look at the veins in the leaf. If they branch out like a tree (dendritic pattern) they are dicots. If the veins are parallel, like the veins in a blade of grass or a corn leaf, it's a monocot. The first link is a pic of an oak leaf, which is a dicot. The second link is a pic of the parallel veins of a corn leaf. Also, dicot flowers tend to have flower parts in multiples of four or five (petals, stamens, sepals). Monocot flowers tend to have flower parts in multiples of threes.
Answers:DICOTS Dicot plants differ in structures of their seeds, foliage and flowers, from the monocotyledons. They can be easily distinguished by studying their distinct characteristics. Let us learn about each feature of the dicot plants in detail. Seed The most important difference between the monocots and dicots is the number of embryonic leaves, or cotyledons. There are two cotyledons in the seeds of dicot plants that actually emerge above the soil, when the seed germinates. They then turn green and form the first two leaves of the new plant. Leaves The leaves of the dicot plants have veins that form a branched pattern, unlike monocots, in which they run parallel. The veins are actually netted or webbed on the whole surface of the leaf, in dicot plants. Vascular Structure The stems of dicot plants have vascular bundles, consisting of two structures, the xylem and the phloem. The xylem helps to transport water and minerals from the root to the other parts of the plant and the phloem transports food that is made in the leaves, to the storage organs. These vascular bundles are arranged in a circular manner around the edge of the stem. Flowers The flower parts of dicot plants are usually present in fours or fives. Sometimes, they are found in multiples of either numbers. These flower parts include petals, sepals and pistils, or the reproductive parts of the plant. Roots Roots of dicot plants are often fibrous and branched. They branch out in many smaller parts that form a dense network of root system, unlike monocot plants, that have a tap root system. SEE HERE http://library.thinkquest.org/3715/seeds.html http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html http://www.howtoorganicgarden.com/new_page_2.htm