Lignin

lignin
The chief noncarbohydrate constituent of wood, a polymer that functions as a natural binder and support for the cellulose fibers of woody plants.
GardenWeb Glossary of Botanical Terms
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Lignin - Hard material is cellulose plant cell walls used for support in terrestrial plants.
Lime
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lignin A tough, durable plant substance deposited in cell walls, especially in wood.
lime A rock powder consisting primarily of calcium carbonate. Used to raise soil pH (decrease acidity).

to convert into wood or woody tissue; to become wood or woody by chemical and physical changes in the cell walls that convert some or all of the constituents into lignin or lignocellulose.lignin search for term- n.

The stem can be soft or hard, by presence of lignin.
So, in this concept, small plants, like purslanes and grasses, and even large and aparently woody plants, like palms and yuccas, can be considered herbs, since they don´t do secondary growth.

Cedar and Redwood will tend to get lighter after initial installation as the colored extractives and lignin are removed. Light wood, such as pine and fir, tend to get darker. Rain washes the degraded wood materials from the surface.

The organic mulches highest in lignin--an organic compound in woody plants--take the longest to break down. Bark has more lignin than wood, so bark mulches last longer than wood mulches. Cypress and pine straw last almost as long as pine bark.

Tree leaves and turfgrass blades are different. Grass clippings are mostly water and with very little lignin, they can break down quickly. Tree leaves that fall to the ground are dried out and high in lignin, so they won't decompose as readily.

The theory is that as barley straw decomposes in water, it releases lignins. These are oxidized into humic acids when exposed to oxygen. If sunlight then shines on the humic substances, hydrogen peroxide is formed.

Their main contribution to a compost pile is to break down cellulose and lignin, after faster acting bacteria make inroads on them. They prefer cooler temperatures (70 to 75º F) and easily digested food sources.

Each kind of humus is a complex mixture of biochemical compounds in a colloidal or gel-like form, including plant waxes and lignins, and the gums and starches transformed by bacteria and fungi as they break them down.

Intermediate between these two stages is a stage of development called the semihardwood stage in which the wood has fully expanded in size but only the cellulose of the cell wall has formed, not the lignin which makes the wood truly rigid and woody.

See also: Plant, Water, Woody, Organic, Grow