Historically, vascular plants were known as "higher plants", as it was believed that they were further
evolved than other plants due to being more complex organisms. However, this is an antiquated remnant of the obsolete
scala naturae, and the term is generally considered to be unscientific.[10]
Characteristics
Botanists define vascular plants by three primary characteristics:
Vascular plants have
vascular tissues which distribute resources through the plant. Two kinds of vascular tissue occur in plants:
xylem and
phloem. 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 a
vascular bundle.[11] The
evolution of vascular tissue in plants allowed them to evolve to larger sizes than
non-vascular plants, which lack these specialized conducting tissues and are thereby restricted to relatively small sizes.
In vascular plants, the principal
generation or phase is the sporophyte, which produces
spores and is
diploid (having two sets of
chromosomes per cell). (By contrast, the principal generation phase in non-vascular plants is the gametophyte, which produces
gametes and is
haploid - with one set of chromosomes per cell.)
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 a
phylum 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 farther. 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]
Phylogeny
A proposed phylogeny of the vascular plants after Kenrick and Crane 1997[12] is as follows, with modification to the gymnosperms from Christenhusz et al. (2011a),[13] Pteridophyta from Smith et al.[14] and lycophytes and ferns by Christenhusz et al. (2011b) [15] The cladogram distinguishes the
rhyniophytes from the "true" tracheophytes, the eutracheophytes.[12]
This phylogeny is supported by several molecular studies.[14][16][17] Other researchers state that taking fossils into account leads to different conclusions, for example that the ferns (Pteridophyta) are not monophyletic.[18]
Hao and Xue presented an alternative phylogeny in 2013 for pre-
euphyllophyte plants.[19]
Water and
nutrients in the form of inorganic solutes are drawn up from the soil by the roots and transported throughout the plant by the
xylem. Organic compounds such as
sucrose produced by
photosynthesis in leaves are distributed by the
phloemsieve-tube elements.
The xylem consists of
vessels in
flowering plants and of
tracheids in other vascular plants. Xylem cells are dead hard-walled hollow cells arranged to form files of tubes that function in the transport of water. A tracheid cell-wall usually contains the polymer
lignin.
The phloem, on the other hand, consists of living cells called
sieve-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 as
nuclei or
ribosomes, but cells next to them, the
companion cells, function to keep the sieve-tube members alive.
Transpiration
The most abundant
compound in all plants, as in all cellular organisms, is
water, which has an important structural role and a vital role in
plant metabolism.
Transpiration is the main process of water movement within plant tissues. Plants constantly transpire water through their
stomata to the atmosphere and replace that water with soil moisture taken up by their roots. The movement of
water out of the leaf stomata sets up a transpiration pull or tension in the water-column in the xylem vessels or tracheids. The pull is the result of water
surface tension within the cell walls of the
mesophyll cells, from the surfaces of which evaporation takes place when the stomata are open.
Hydrogen bonds exist between water
molecules, 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 via
osmosis. Consequently, transpiration requires the plant to expend very little energy on water movement. Transpiration assists the plant in absorbing nutrients from the soil as soluble
salts. 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.[20]
Absorption
Living root cells passively absorb water in the absence of transpiration pull via
osmosis creating root pressure. It is possible for there to be no
evapotranspiration and therefore no pull of water towards the shoots and leaves. This is usually due to high temperatures, high
humidity, darkness or drought.[citation needed]
Conduction
Xylem is the water-conducting tissue, and secondary xylem provides the raw material for the forest products industry.[21]Xylem and
phloem tissues each play a part in the conduction processes within plants. Sugars are conducted throughout the plant in the phloem; water and other nutrients through the xylem. Conduction occurs from a source to a sink for each separate nutrient. Sugars are produced in the leaves (a source) by
photosynthesis 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 allow
cell division and growth.[22][23][24]
^
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.
^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.
S2CID4367248.
^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.
S2CID86172890.