Tracheophyta

Core Defining Characteristics

Tracheophyta, or vascular plants, are defined by botanists through three core primary characteristics. First, vascular plants possess specialized vascular tissues that distribute resources throughout the plant body.

Vascular Tissue Types

There are two types of vascular tissue: xylem and phloem. These two tissues are closely associated, typically located immediately next to one another within the plant.

Vascular Bundle Definition

A paired adjacent strand of one xylem and one phloem is called a vascular bundle. The evolution of vascular tissue allowed vascular plants to grow to larger sizes than non-vascular plants, which lack these specialized conducting tissues and are therefore restricted to relatively small sizes.

Sporophyte Generation Trait

Second, the main life cycle generation or phase in vascular plants is the diploid sporophyte, which produces spores and has two sets of chromosomes per cell. In contrast, the dominant generation in non-vascular plants is the haploid gametophyte, which produces gametes and carries one set of chromosomes per cell.

Organ Structure Trait

Third, vascular plants have true roots, leaves, and stems, though some groups have secondarily lost one or more of these traits.

Taxonomic Classification

In 1998, Cavalier-Smith treated Tracheophyta as a botanical phylum or division, defined by the Latin phrase "facies diploida xylem et phloem instructa", which translates to "diploid phase with xylem and phloem".

Evolution of Diploid Dominance

One proposed mechanism for the evolutionary shift from a dominant haploid generation to a dominant diploid generation is that more complex diploid structures enable more efficient spore dispersal. Elongation of the spore stalk allowed plants to produce more spores, release them from a greater height, and spread them over longer distances.

Associated Evolutionary Adaptations

These related developments include a larger photosynthetic area for the spore-bearing structure, the ability to grow independent roots, a woody structure for support, and increased branching.

Sexual Reproduction Mechanism

Sexual reproduction in vascular land plants relies on the process of meiosis. Meiosis provides direct DNA repair function to address DNA damage, including oxidative DNA damage, in germline reproductive tissues.

Xylem Nutrient Transport

In terms of nutrient distribution, water and nutrients in the form of inorganic solutes are absorbed from the soil by the roots, then transported throughout the plant by xylem.

Phloem Nutrient Transport

Organic compounds such as sucrose, which are produced via photosynthesis in leaves, are distributed by phloem sieve-tube elements.

Xylem Structural Composition

Xylem is made up of vessels in flowering plants, and of tracheids in all other vascular plants. Xylem cells are dead, hard-walled, hollow cells arranged into files of tubes that carry out water transport.

Tracheid Cell Traits

A tracheid cell wall usually contains the polymer lignin.

Phloem Structural Composition

Phloem, by contrast, is made up of living cells called sieve-tube members. Pored sieve plates sit between adjacent sieve-tube members to allow molecules to pass through.

Sieve-Tube Member Support

Sieve-tube members lack structures such as nuclei or ribosomes, but adjacent companion cells maintain the function and survival of sieve-tube members.