Vascular tissues do not only fulfil supporting functions. Their conductive functions are more important. They consist of water-conducting xylem and food-conducting phloem. Only the xylem has still supporting functions. Xylem and phloem are combined in a structure that is called vascular bundle.
Vascular tissues transport water, dissolved salts, products of photosynthesis (assimilates), growth regulators like phytohormones and, last but not least, harmful substances and parasites like viruses or mycoplasmas. It is commonly distinguished between the elements of water conduct, the xylem, and those of assimilate conduct, the phloem. Both are combined in vascular bundles. Vascular bundles contain mostly dead cells. In contrast to all other tissues of the actual topic, the phloem, that will be reviewed here together with the xylem has has no supporting function whatsoever.
The lignified cell walls of the xylem elements will at the moment, rather simplifyingly, be called wood. They can last for thousands of years. Their petrified states are ideal evidences for the first occurrence and the successive spreading of the great taxonomical groups of terrestrial plants.
Genuine vascular bundles are the name-giving feature of vascular plants (Trachaeophtes: Pteridophytes and Phanerogames). Thallophytes have at most first signs of a conductive system, which is not homologous to the respective tissues in vascular plants. Primitive variations of conductive elements appear also in mosses.
The occurring cell types and the arrangement of the vascular bundles within shoots and roots are reliable features for the characterization of individual classes of plants. The xylem of nearly all angiosperms contains three types of cells:
wood vessels
tracheids and
the already mentioned xylem fibres.
Most gymnosperms lack wood vessels. The few exceptions point out the relationship between gymnosperms and angiosperms. The vascular bundles of monocots seem to run through the shoot without any obvious order, while those of dicots and many gymnosperms are arranged in peripheral rings. Xylem and phloem are separated by a meristematic tissue, the vascular cambium. It provides the means for secondary growth. The vascular bundles of pteridophytes and the roots of flowering plants are located centrally.
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