Historically, vascular plants were known as "higher plants", as it was believed that they were furtherevolved than other plants due to being more complex organisms. However, this is an antiquated remnant of the obsoletescala naturae, and the term is generally considered to be unscientific.[13]
Botanists define vascular plants by three primary characteristics:
Vascular plants havevascular tissues which distribute resources through the plant. Two kinds of vascular tissue occur in plants:xylem andphloem. Phloem and xylem are closely associated with one another and are typically located immediately adjacent to each other in the plant. The combination of one xylem and one phloem strand adjacent to each other is known as avascular bundle.[14] Theevolution of vascular tissue in plants allowed them to evolve to larger sizes thannon-vascular plants, which lack these specialized conducting tissues and are thereby restricted to relatively small sizes.
Vascular plants have true roots, leaves, and stems, even if some groups have secondarily lost one or more of these traits.
Cavalier-Smith (1998) treated the Tracheophyta as aphylum or botanical division encompassing two of these characteristics defined by the Latin phrase "facies diploida xylem et phloem instructa" (diploid phase with xylem and phloem).[4]: 251
One possible mechanism for the presumed evolution from emphasis on haploid generation to emphasis on diploid generation is the greater efficiency in spore dispersal with more complex diploid structures. Elaboration of the spore stalk enabled the production of more spores and the development of the ability to release them higher and to broadcast them further. Such developments may include more photosynthetic area for the spore-bearing structure, the ability to grow independent roots, woody structure for support, and more branching.[citation needed]
Sexual reproduction in vascular land plants involves the process of meiosis. Meiosis provides a directDNA repair capability for dealing withDNA damages, including oxidative DNA damages, ingermline reproductive tissues.[15]
A proposed phylogeny of the vascular plants after Kenrick and Crane 1997[16] is as follows, with modification to the gymnosperms from Christenhuszet al. (2011a),[17] Pteridophyta from Smithet al.[18] and lycophytes and ferns by Christenhuszet al. (2011b)[19] The cladogram distinguishes therhyniophytes from the "true" tracheophytes, the eutracheophytes.[16]
This phylogeny is supported by several molecular studies.[18][20][21] Other researchers state that taking fossils into account leads to different conclusions, for example that the ferns (Pteridophyta) are not monophyletic.[22]
Hao and Xue presented an alternative phylogeny in 2013 for pre-euphyllophyte plants.[23]
Thexylem consists ofvessels inflowering plants and oftracheids in other vascular plants. Xylem cells are dead, hard-walled hollow cells arranged to form files of tubes that function in water transport. A tracheid cell wall usually contains the polymerlignin.[citation needed]
Thephloem, on the other hand, consists of living cells calledsieve-tube members. Between the sieve-tube members are sieve plates, which have pores to allow molecules to pass through. Sieve-tube members lack such organs asnuclei orribosomes, but cells next to them, thecompanion cells, function to keep the sieve-tube members alive.[citation needed]
The most abundantcompound in all plants, as in all cellular organisms, iswater, which has an important structural role and a vital role inplant metabolism.Transpiration is the main process of water movement within plant tissues. Plants constantly transpire water through theirstomata to the atmosphere and replace that water with soil moisture taken up by their roots. When the stomata are closed at night, water pressure can build up in the plant. Excess water is excreted through pores known ashydathodes.[24] The movement ofwater out of the leaf stomata sets up transpiration pull or tension in the water column in the xylem vessels or tracheids. The pull is the result of watersurface tension within the cell walls of themesophyll cells, from the surfaces of which evaporation takes place when the stomata are open.Hydrogen bonds exist between watermolecules, causing them to line up; as the molecules at the top of the plant evaporate, each pulls the next one up to replace it, which in turn pulls on the next one in line. The draw of water upwards may be entirely passive and can be assisted by the movement of water into the roots viaosmosis. Consequently, transpiration requires the plant to expend very little energy on water movement. Transpiration assists the plant in absorbing nutrients from the soil as solublesalts. Transpiration plays an important role in the absorption of nutrients from the soil as soluble salts are transported along with the water from the soil to the leaves. Plants can adjust their transpiration rate to optimize the balance between water loss and nutrient absorption.[25]
Living root cells passively absorb water. Pressure within the root increases when transpiration demand viaosmosis is low and decreases when water demand is high. No water movement towards the shoots and leaves occurs whenevapotranspiration is absent. This condition is associated with high temperature, highhumidity, darkness, and drought.[citation needed]
Xylem is the water-conducting tissue, and the secondary xylem provides the raw material for the forest products industry.[26] Xylem andphloem tissues each play a part in the conduction processes within plants. Sugars are conveyed throughout the plant in the phloem; water and other nutrients pass through the xylem. Conduction occurs from a source to a sink for each separate nutrient. Sugars are produced in the leaves (a source) byphotosynthesis and transported to the growing shoots and roots (sinks) for use in growth,cellular respiration or storage. Minerals are absorbed in the roots (a source) and transported to the shoots to allowcell division and growth.[27][28][29]
^abKenrick, Paul; Crane, Peter R. (1997).The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D.C.: Smithsonian Institution Press.ISBN1-56098-730-8.
^Christenhusz, Maarten J. M.; Reveal, James L.; Farjon, Aljos; Gardner, Martin F.; Mill, R.R.; Chase, Mark W. (2011). "A new classification and linear sequence of extant gymnosperms".Phytotaxa.19:55–70.doi:10.11646/phytotaxa.19.1.3.
^abSmith, Alan R.; Pryer, Kathleen M.; Schuettpelz, E.; Korall, P.; Schneider, H.; Wolf, Paul G. (2006). "A classification for extant ferns".Taxon.55 (3):705–731.doi:10.2307/25065646.JSTOR25065646.
^Christenhusz, Maarten J. M.; Zhang, Xian-Chun; Schneider, Harald (2011). "A linear sequence of extant families and genera of lycophytes and ferns".Phytotaxa.19:7–54.doi:10.11646/phytotaxa.19.1.2.
^Pryer, K. M.; Schneider, H.; Smith, A. R.; Cranfill, R.; Wolf, P. G.; Hunt, J. S.; Sipes, S. D. (2001). "Horsetails and ferns are a monophyletic group and the closest living relatives to seed plants".Nature.409 (6820):618–22.Bibcode:2001Natur.409..618S.doi:10.1038/35054555.PMID11214320.
^Pryer, K. M.; Schuettpelz, E.; Wolf, P. G.; Schneider, H.; Smith, A. R.; Cranfill, R. (2004). "Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences".American Journal of Botany.91 (10):1582–1598.doi:10.3732/ajb.91.10.1582.PMID21652310.
^Rothwell, G. W. & Nixon, K. C. (2006). "How Does the Inclusion of Fossil Data Change Our Conclusions about the Phylogenetic History of Euphyllophytes?".International Journal of Plant Sciences.167 (3):737–749.doi:10.1086/503298.
^Hao, Shougang; Xue, Jinzhuang (2013).The Early Devonian Posongchong Flora of Yunnan: A Contribution to an Understanding of the Evolution and Early Diversification of Vascular Plants. Science Press.ISBN978-7-03-036616-0.[page needed]
^Doyle, James A. (1998). "Phylogeny of Vascular Plants".Annual Review of Ecology and Systematics.29 (1):567–599.doi:10.1146/annurev.ecolsys.29.1.567.
^Heijmans, Monique M. P. D.; Arp, Wim J.; Berendse, Frank (October 2001). "Effects of elevated CO 2 and vascular plants on evapotranspiration in bog vegetation: EVAPOTRANSPIRATION IN BOG VEGETATION".Global Change Biology.7 (7):817–827.doi:10.1046/j.1354-1013.2001.00440.x.
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