Lignin is a three dimensional phenolic polymer network. Chemical pulping and bleaching processes selectively remove the lignin without significantly degrading the cellulose fibres. These materials include plant hormones, resin and fatty acids along with other substances that help the tree grow and resist disease and pests. These substances are highly toxic to aquatic life and account for much of the acute toxicity of pulp mill effluent. Softwood trees are conifers - e.g., southern pine, Douglas fir, spruce.
Hardwood trees lose their leaves every year.
Examples include birch, aspen, red gum. Softwood fibres with their length and coarseness are generally used to provide strength to a sheet of paper. Hardwood fibres, being finer and more conformable, give a sheet of paper its smooth printing surface and opacity. Hardwood fibres are also easier to bleach to high brightness because they have less lignin. Paper generally consists of a blend of hardwood and softwood pulps to meet the strength and printing surface demands of the customer.