Vascular plant

Vascular plants Temporal range: Ma[1][2] PreꞒ Ꞓ O S D C P T J K Pg N
Common lady-fern , a non-seed-bearing plant
Lemon basil, a seed-bearing plant
Scientific classification
Kingdom:	Plantae
Clade:	Embryophytes
Clade:	Polysporangiophytes
Clade:	Tracheophytes Sinnott, 1935[3] ex Cavalier-Smith, 1998[4]
Divisions † Extinct
Non-seed-bearing plants †Cooksonia †Rhyniophyta †Zosterophyllophyta Lycopodiophyta † Trimerophytophyta Polypodiophyta † Progymnospermophyta Superdivision Spermatophyta †Pteridospermatophyta Pinophyta Cycadophyta Ginkgophyta Gnetophyta Magnoliophyta (angiosperms) †Bennettitales

Vascular plants (from

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:

1. 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.
2. In vascular plants, the principal
   haploid
   - with one set of chromosomes per cell.)
3. 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]

Polysporangiophytes † Tracheophytes †Cooksoniaceae †Aglaophyton † Rhyniopsida †Catenalis †Aberlemnia †Hsuaceae † Eutracheophytes †Adoketophyton †? Barinophytopsida † Zosterophyllopsida Microphylls †Hicklingia †Gumuia † Nothia †Zosterophyllum deciduum †Yunia Euphyllophytes †Eophyllophyton †Trimerophytopsida Megaphylls Moniliformopses †Ibyka †Pauthecophyton †Cladoxylopsida Polypodiopsida Radiatopses †Celatheca †Pertica Lignophytes †Progymnosperms (paraphyletic)

Rhyniopsids Renalioids

Nutrient distribution

Xylem elements in the shoot of a fig tree (Ficus alba), crushed in hydrochloric acid

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 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

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]

See also

• Fern allies
• Bryophytes
• Non-vascular plant
• Pteridophyte

References