Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from the Greek ''trācheia''), form a large group of plants ( 300,000 accepted known species)
that are defined as land plants
with lignified tissues
) for conducting water and minerals throughout the plant. They also have a specialized non-lignified tissue (the phloem
) to conduct products of photosynthesis
. Vascular plants include the clubmoss
s (including conifer
s) and angiosperms
s). Scientific names for the group include Tracheophyta,
and Equisetopsida ''sensu lato''
. Some early land plants (the rhyniophyte
s) had less developed vascular tissue; the term eutracheophyte has been used for all other vascular plants.
Botanists define vascular plants by three primary characteristics:
# Vascular plants have vascular tissue
s which distribute resources through the plant. Two kinds of vascular tissue occur in plants: xylem
. 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
. The evolution
of vascular tissue in plants allowed them to evolve to larger sizes than non-vascular plant
s, which lack these specialized conducting tissues and are thereby restricted to relatively small sizes.
# In vascular plants, the principal generation phase
is the ''sporophyte
'', which produces spore
s 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).
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.
A proposed phylogeny of the vascular plants after Kenrick and Crane 1997
is as follows, with modification to the gymnosperms from Christenhusz ''et al.'' (2011a),
Pteridophyta from Smith ''et al.''
and lycophytes and ferns by Christenhusz ''et al.'' (2011b)
The cladogram distinguishes the rhyniophyte
s from the "true" tracheophytes, the eutracheophytes.
This phylogeny is supported by several molecular studies.] Other researchers state that taking fossils into account leads to different conclusions, for example that the ferns (Pteridophyta) are not monophyletic.
Hao and Xue presented an alternative phylogeny in 2013 for pre-euphyllophyte plants.
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 phloem sieve tube elements.
The xylem consists of vessels in flowering plants and tracheids in other vascular plants, which are dead hard-walled hollow cells arranged to form files of tubes that function in water transport. A tracheid cell wall usually contains the polymer lignin. The phloem, however, 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.
The most abundant compound in all plants, as in all cellular organisms, is water, which serves an important structural role and a vital role in plant metabolism. Transpiration is the main process of water movement within plant tissues. Water is constantly transpired from the plant through its stomata to the atmosphere and replaced by soil water taken up by the roots. The movement of water out of the leaf stomata creates 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 very little energy to be used by the plant. Transpiration assists the plant in absorbing nutrients from the soil as soluble salts.
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.
Xylem and phloem tissues are involved 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.
[Chapters 5, 6 and 10 Taiz and Zeiger ''Plant Physiology'' 3rd Edition SINAUER 2002]
* , in
“Higher plants” or “vascular plants”?
Category:Extant Silurian first appearances