Dicots

2018-05-09T12:19:28Z

Nkstepha: /* Definition */

== Definition ==

[[File:Sunflower.jpg|frame|Sunflower, reference 2]]

A dicotyledon (commonly referred to as a dicot) is an angiospermous plant with two cotyledons and having an exogenous manner of growth. Cotyledons are the “seed leaves” that absorb nutrients within the seed until the plant can produce true leaves and begin photosynthesis. The term dicotyledon refers to the group containing seeds with two cotyledons, rather than one. Monocotyledons are the remaining group that contains seeds with only one cotyledon. There are about 175,000 known species of dicots, about half of which are woody. These plants show a yearly increase in stem diameter due to the production of new tissue by the cambium, which is a layer of cells that continue to divide throughout the life of a plant.

== History ==

Angiosperms have traditionally been divided into two major classes, the Magnoliopsida (Dicots), and the Liliopsda (Monocots), although botanists have not always recognized these as the two fundamental groups of angiosperms. Theophrastus was first credited with recognizing the difference between the two groups around 370 BC, but classification of the plants based on overall growth form was not established until the 1600s.

Recent studies using two known fossil nodes and the Li–Tanimura method, estimate that monocots branched off from dicots 140–150 million years ago during the late Jurassic–early Cretaceous period. This is around 50 million years younger than previous estimates that were based solely on the molecular clock hypothesis. This study also estimates that the core eudicots diverged 100–115 million years ago during the Cretaceous period, indicating that both the divergence of monocots from dicots, as well as the core eudicot’s age are older than their respective fossil records.

== Formation and Differentiation from Monocots ==

Once an angiosperm has been pollenated, two sperm will drop into the embryonic sac where one will fuse with the egg to form a zygote, and the other will fuse with the two nuclei of the central cell and become an endosperm nucleus. This is referred to as double fertilization, as the true fertilization is accompanied by another fusion process that resembles fertilization. This type of double fertilization is a trait unique to angiosperms. The zygote is now diploid, and the endosperm nucleus is triploid. The endosperm nucleus undergoes mitosis to form the endosperm of the seed, a nutrient-rich tissue utilized by the developing embryo and germinating seed.

The zygote then undergoes a series of mitotic divisions to form an undifferentiated embryo. Differentiation of the embryo requires the development of a radicle (apical meristem) next to the suspensor from where the root will begin growing, and the development of one cotyledon (in monocotyledons) or two cotyledons (in dicotyledons) at the opposite end from the suspensor. A shoot apical meristem is the site of stem differentiation, differentiating between the two cotyledons or forming next to the single cotyledon.

While the primary difference between these two groups lies in their seed structure, there are several additional differences that designate a flowering plant as either mono or dicot.

'''Roots'''

The roots of monocots branch off in many different directions resembling a fibrous web. Monocot roots remain primarily in the upper level of soil and do not dig as deep down as do dicot roots.
Dicots have one main “taproot” off of which smaller roots branch off. This structure allows the root to grow down further rather than expending energy spreading outwards.

[[File:Dicots.png|frame|]]

'''Pollen Structure'''

Monocots retain the first angiosperm’s pollen structure which contained a single pore through it’s outer layer, known as monosulcate. Dicots descended from a plant which contained three pores in it’s pollen, known as triporate.

'''Stems'''

The vascular bundles in monocots are arranged sporadically throughout the stem in no particular pattern.
Dicots all contain vascular bundles that are arranged in a ring around the outer edge of the vascular tissue.
Vascular tissue can be thought of as the circulatory system of a plant, and therefore the distinction in bundles is important to note.

'''Leaves'''

In monocot plants, leaves are characterized by parallel veins and typically thin leaves. The leaf structure of dicots is branched or webbed veins throughout the leaf structure. This is not always the way things are because some monocots have been seen to have webbed veins.

'''Flowers'''

In monocot plants, the number of petals, stamens, or other floral parts will typically be a number divisible by three; usually either three or six. Dicot flowers on the other hand, tend to have parts in multiples of four or five. This character is not always reliable, and can be misleading as certain flowers may be lacking parts.

== References ==

1. Barry, P. E., & Stevens, P. (2018, March 6). Angiosperm. In Encyclopædia Britannica. Retrieved March 7, 2018, from https://www.britannica.com/plant/angiosperm/Fertilization-and-embryogenesis#ref73133

2. Brodie, C. (2005, November). Sunflower. [Illustration]. Retrieved from http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artnov05/cbpaint.html

3. Dicotyledon. (2016, September 5). In Encyclopædia Britannica. Retrieved March 7, 2018, from https://www.britannica.com/plant/dicotyledon

4. Speer, B. R. (1995, November 23). Monocots versus Dicots The Two Classes of Flowering Plants. Retrieved March 4, 2018, from ump.berkeley.edu website: http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html

5. dicotyledon. (n.d.). Dictionary.com Unabridged. Retrieved March 8, 2018 from Dictionary.com website http://www.dictionary.com/browse/dicotyledon

6. Phelan, Jay. What Is Life? A Guide to Biology with Physiology. New York: W.H. Freeman Custom Publishing, 2011. Print.

7. Drinnan A.N., Crane P.R., Hoot S.B. (1994) Patterns of floral evolution in the early diversification of non-magnoliid dicotyledons (eudicots). In: Endress P.K., Friis E.M. (eds) Early Evolution of Flowers. Plant Systematics and Evolution Supplement 8, vol 8. Springer, Vienna

2018-05-09T11:46:21Z

Nkstepha: /* Environmental Impacts */

== Description ==

Springtails are members of the Collembola family and are relatively small (typically less than 6 millimeters in length), with as many as six abdominal segments [1]. They have a tubular appendage called a collophore, which protrudes from the first abdominal segment. The collophore used to be thought to be a stabilizing mechanism for the collembola when it jumped by sticking to the surface on which it landed. More recent research has concluded that the collophore is used in osmoregulation, water intake, and excretion [2].

[[File:Springtail.jpg|thumb|]]

Collembola that live in the upper soil layers are often referred to as Springtails because of a tail-like appendage found among most species, called the furcula. It is a forked appendage attached to the fourth segment by a structure called the retinaculum and is used for jumping when the animal is threatened; it is not used for normal locomotion [3]. In as little as 18 milliseconds the furcula can be released from the retinaculum, snapping against the substrate and flinging the springtail into the air [4]. A reason that this mechanism is not used in typical locomotion is that it’s direction is very unpredictable. When the furcula is released, the springtail is sent tumbling through the air on an arbitrary trajectory, landing randomly [3].

Springtails are able to reduce their body size by up to 30% through genetically controlled molting if temperatures are high enough. Warmer conditions increase energy needs, as well as metabolic rates, therefore a smaller body size comes with many advantages to the organism [5].

== Habitat & Distribution ==

Springtails are typically found in leaf litter and other decaying organic material [6]. They mainly consume fungal hyphae and spores, but can also consume bacteria, plant material and pollen, minerals, and animal remains [7].

They are one of the most abundant macroscopic animals in the world, with estimates of 100,000 individuals per square meter of ground where soil and [[moss]] occur [8]. The only other amimals with global populations of a similar size are thought to be [[nematodes]], crustaceans, and [[mites]]. Most springtails are difficult to see with the naked eye with few exceptions, including the snow flea [9]. Several species of springtail climb trees, and form a dominant component of canopy faunas. These species are typically smaller than soil populations of the same species by 1-2 orders of magnitude [10].

[[File:Springtail2.jpg|thumb|left]]

Overall, springtails are very susceptible to desiccation due to the structure of their respiratory system, although sensitivity to drought varies from species to species [11].

The distribution of springtail species is affected by environmental factors such as soil acidity, moisture, and light. Altitudinal changes in species distribution can be at least partially explained by increased acidity at higher elevation, and moisture requirements explain why some species cannot live above ground, or retreat into the soil during dry seasons [12] [13].

== Evolution ==

Springtails have been found as far back as the Early Devonian in the fossil record. The fossil of Rhyniella praecursor, the oldest terrestrial arthropod, was found in the Rhynie chert of Scotland and is over 400 million years old [14]. Fossil collembola are very rare, and most are preserved in amber. Even these are rare, and all but one of the fossils from the Cretaceous belongs to extinct genera [15].

In total, there are around 3,600 different species of springtail [16].

== Environmental Indicators ==

Springtails are commonly used in laboratory tests to detect soil pollution at it’s early stages. Scientists have performed both acute and chronic toxicity tests, and primarily use the parthenogenetic isotomid Folsomia candida [17]. Avoidance tests have additionally been performed, and are complementary to toxicity tests. Avoidance tests are quicker, less expensive, and more environmentally reliable, as in real instances of pollution, Collembola move far away from pollution spots [18]. Collembola are useful bio-indicators of soil quality. Studies have found the jumping ability of springtails can be used to evaluate the quality of Cu- and Ni-polluted soils [19].

== References ==
1. Davies, W. Maldwyn (1927). "On the tracheal system of Collembola, with special reference to that of Sminthurus viridis, Lubb" (PDF). Quarterly Journal of Microscopical Science. 71 (281): 15–30.

2. Eisenbeis, G., 1982. Physiological absorption of liquid water by Collembola: absorption by the ventral tube at different salinities. Journal of Insect Physiology 28:11–20.

3. Christian, E., 1978. The jump of the springtails. Naturwissenschaften 65:495-496.

4. Piper, Ross (2007). Extraordinary animals: an encyclopedia of curious and unusual animals. Santa Barbara, California: Greenwood Press.

5. "The incredible shrinking springtail". Science. 341 (6149): 945. 30 August 2013. doi:10.1126/science.341.6149.945-a.

6. Hopkin, Stephen P. (1997). "The biology of the Collembola (springtails): the most abundant insects in the world" (PDF). Natural History Museum.

7. Chen, Benrong; Snider, Richard J. & Snider, Renate M. (1996). "Food consumption by Collembola from northern Michigan deciduous forest" (PDF). Pedobiologia. 40 (2): 149–161.

8. Ponge, Jean-François; Arpin, Pierre; Sondag, Francis & Delecour, Ferdinand (1997). "Soil fauna and site assessment in beech stands of the Belgian Ardennes" (PDF). Canadian Journal of Forest Research. 27 (12): 2053–2064. doi:10.1139/cjfr-27-12-2053.

9. Island Creek Elementary School. "Snow Flea. Hypogastrura nivicola". Study of Northern Virginia Ecology. Fairfax County Public Schools.

10. Shaw, Peter; Ozanne, Claire; Speight, Martin & Palmer, Imogen (2007). "Edge effects and arboreal Collembola in coniferous plantations" (PDF). Pedobiologia. 51 (4): 287–293. doi:10.1016/j.pedobi.2007.04.010.

11. Nickerl, Julia; Helbig, Ralf; Schulz, Hans-Jürgen; Werner, Carsten & Neinhuis, Christoph (2013). "Diversity and potential correlations to the function of Collembola cuticle structures" (PDF). Zoomorphology. 132 (2): 183–195. doi:10.1007/s00435-012-0181-0.

12.Loranger, Gladys; Bandyopadhyaya, Ipsa; Razaka, Barbara & Ponge, Jean-François (2001). "Does soil acidity explain altitudinal sequences in collembolan communities?" (PDF). Soil Biology and Biochemistry. 33 (3): 381–393. doi:10.1016/S0038-0717(00)00153-X.

13. Detsis, Vassilis (2000). "Vertical distribution of Collembola in deciduous forests under Mediterranean climatic conditions" (PDF). Belgian Journal of Zoology. 130 (Supplement 1): 57–61.

14. Daly, Howell V.; Doyen, John T. & Purcell, Alexander H. (1998). Introduction to insect biology and diversity (2nd ed.). New York: Oxford University Press. ISBN 0-19-510033-6.

15. Mari Mutt, José A. (1983). "Collembola in amber from the Dominican Republic" (PDF). Proceedings of the Entomological Society of Washington. 85 (3): 575–587.

16. Koehler, Philip G.; Aparicio, M. L. & Pfiester, Margaret (July 2011). "Springtails" (PDF). Gainesville, Florida: University of Florida IFAS Extension.

17. Fountain, Michelle T. & Hopkin, Steve P. (2001). "Continuous monitoring of Folsomia candida (Insecta: Collembola) in a metal exposure test" (PDF). Ecotoxicology and Environmental Safety. 48 (3): 275–286. doi:10.1006/eesa.2000.2007.

18. Chauvat, Matthieu & Ponge, Jean-François (2002). "Colonization of heavy metal-polluted soils by collembola: preliminary experiments in compartmented boxes" (PDF). Applied Soil Ecology. 21 (2): 91–106. doi:10.1016/S0929-1393(02)00087-2.

19. Kim, Shin Woong & An, Youn-Joo (2014). "Jumping behavior of the springtail Folsomia candida as a novel soil quality indicator in metal-contaminated soils". Ecological Indicators. 38: 67–71. doi:10.1016/j.ecolind.2013.10.033.