Vascular plant

Vascular Plants Fossil range: Early Silurian - Recent
Scientific classification
Kingdom: Plantae
Divisions
Non-seed-bearing plants †Rhyniophyta †Zosterophyllophyta Lycopodiophyta †Trimerophytophyta Pteridophyta Superdivision Spermatophyta †Pteridospermatophyta Pinophyta Cycadophyta Ginkgophyta Gnetophyta Magnoliophyta

Vascular plants (also known as tracheophytes or higher plants) are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the ferns, clubmosses, flowering plants, conifers and other gymnosperms. Scientific names for the group include Tracheophyta^[1] and Tracheobionta,^[2] but neither name is very widely used.

Characteristics

Vascular plants are distinguished by two primary characteristics:

1. Vascular plants have vascular tissues, which circulate resources through the plant. This feature allows vascular plants to evolve to a larger size than non-vascular plants, which lack these specialized conducting tissues and are therefore restricted to relatively small sizes.
2. In vascular plants, the principal generation phase is the sporophyte, which is usually diploid with two sets of chromosomes per cell. By contrast, the principal generation phase in non-vascular plants is usually the gametophyte, which is haploid with one set of chromosomes per cell.

Water transport happens in either xylem or phloem: xylem carries water and inorganic solutes upward toward the leaves from the roots, while phloem carries organic solutes throughout the plant. Group of plants having lignified conducting tissue (xylem vessels or tracheids).

Phylogeny

A proposed phylogeny of the vascular plants after Kenrick and Crane^[3] is as follows, with modification to the Pteridophyta from Smith et al.^[4]

Nutrient distribution

Photographs showing xylem elements in the shoot of a fig tree (Ficus alba): crushed in hydrochloric acid, between slides and cover slips.

Nutrients and water from the soil and the organic compound produces in leaves are distributed to specific areas in the plant through the xylem and phloem. The xylem draws water and nutrients up from the roots to the upper sections of the plant's body, and the phloem conducts other materials, such as the glucose produced during photosynthesis, which gives the plant energy to keep growing and seeding.

The xylem consists of tracheids, which are dead hard-walled cells arranged to form tiny tubes to 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.

Movement of nutrients, water, sugars and waste is effected by transpiration, conduction and absorption.

Transpiration

The most abundant compound in most plants is water, serving a large role in the various processes taking place. Transpiration is the main process a plant can call upon to move compounds within its tissues. The basic minerals and nutrients a plant is composed of remain, generally, within the plant. Water, however, is constantly being lost from the plant through its metabolic and photosynthetic processes to the atmosphere.

Water is transpired from the plants leaves via stomata, carried there via leaf veins and vascular bundles within the plants cambium layer. The movement of water out of the leaf stomata creates, when the leaves are considered collectively, a transpiration pull. The pull is created through water surface tension within the plant cells. The draw of water upwards is assisted by the movement of water into the roots via osmosis. This process also assists the plant in absorbing nutrients from the soil as soluble salts, a process known as absorption.

Absorption

Xylem cells move water and nutrient solutions upwards towards other plant organs from the roots and fine root hairs. Living roots cells actively absorb water in the absence of transpiration pull via osmosis creating root pressure. There are times when plants do not have transpiration pull, usually due to lack of light or other environmental elements. Water in the plant tissues may move to the roots to assist in passive absorption.

Conduction

Xylem and phloem tissues are involved in the conduction processes within plants. The movement of foods throughout the plant takes place mainly in the phloem. Plant conduction (food movement) is from an area of high food content, place of manufacture (photosynthesis) or storage, to a place of food utilisation, or from a point of manufacture to storage tissues. Mineral salts are translocated in the xylem tissues.^[5]

See also

• Fern allies
• Non-vascular plant

References