The most striking feature of secondary walls is their loss of plasticity. Progressive depositions of new lamellas thicken the wall while the cell lumen's diameter decreases.
Mostly are secondary walls less hydrated than primary walls and contain less pectins and hemicellulose. Instead are other components deposited, which are sometimes characteristic for certain cell groups or tissues. The best-known deposited substance is perhaps lignin (discovered in 1839 by A. PAYEN as the fraction of cell walls insoluble in acids). It is the basic unit of xylem- and strengthening elements (wood) and consists of polymerized phenylpropane units. The three most important starting compounds are coumaryl alcohol [with an OH-group in position 4 of the phenyl ring], coniferyl alcohol (OH-group in position 4, -OCH3 in position 3) and sinapyl alcohol (OH-group in position 4, -OCH3 group in positions 3 and 5).
The lignins of the single plant groups differ in the percentages of these starting compounds and in the way they are linked. All bonds leading to the formation of a three-dimensional molecular network are covalent. In other words: a simple breaking and subsequent reforming of weak interactions between the individual molecules as we met in the cellulose-hemicellulose complex does not exist here. As a consequence form lignins a network that fulfils all requirements of stability (flexible and tension-proof). The disadvantage is that the bonds are irreversible, a stretching of the wall - and a growth of the cell - are impossible; strongly lignified cells die sooner or later. Only vessel plants contain lignins. Their coming into being (in the early Devonian era) was without doubt one of the most important prerequisites of the evolution of large, upright-growing terrestrial plants. The lignin of pteridophytes consists mainly of coniferyl alcohol polymers, in dicots occur coniferyl and sinapyl alcohol in roughly equal amounts. In the lignins of all plant groups are only trace amounts of coumaryl alcohol found.
No lignins have been found in mosses, instead were different polyphenols and poly-p-hydroxyphenols detected.
Other Strengthening Elements of Higher Plants: Mannanes are a structural element of many seeds, the walls of pollen contain sporopollenin, a polymerization product of carotene.
Many secondary walls contain a wide range of strongly hydrophobic compounds, like suberine, the basic component of cork. Such compounds may on one hand be integral components of the wall itself, but they may, too, be deposited on the wall as solid excretion products (cuticle, wax deposits, etc.).
Beside the structural elements of the wall have its non-structural components to be mentioned. Among them are a number of low molecular weight compounds [dyes, alcohols, terpenes, tannins, etc.], oligosaccharides (and polysaccharides) of different configurations as well as proteins (usually glycoproteins). Some of them participate in recognition processes, like the incompatibility factors at the stigma surface and several carbohydrate-binding lectins. Remarkable are especially two lectins yielded from the walls of Solanaceous species (Solanum tuberosum and Datura stramonium). Both contain a high degree of hydroxyproline that may point at a phylogenetic relation of these lectins with extensin.
Some cell walls, like that of some seeds, become mucilaginous. The process is based on a partial enzymatic degradation of the structural polymers. Mucus has usually a gel-like consistency, but on a molecular level are they often molecular mosaics of a distinct molecular architecture. E. SCHNEPF and G. DEICHGRÄBER (Cytology, Universität Heidelberg), for example, discovered that the mucilage in the epidermal cells of the seeds of Ruellia (Acanthaceae) consist of long fibrils and an amorphous matrix. Since these fibrils can be stained with calcofluor white and aniline blue was it naturally assumed that they contain beta 1 > 4 and / or beta 1 > 3 glucans (cellulose and / or callose). The orientation of these fibrils is determined by microtubuli of the same orientation within the fibril-producing cells.
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de