Botany strengthen the secondary cell walls of xylem tracheids and vessels to keep them from collapsing when a plant sucks water through them under water stress. Lignin is also used in other cell types like sclerenchyma fibres that provide structural support for a plant and is a major constituent of wood. Sporopollenin is a chemically resistant polymer found in the outer cell walls of spores and pollen of land plants responsible for the survival of early land plant spores and the pollen of seed plants in the fossil record. It is widely regarded as a marker for the start of land
Hilum (biology) such a mark, and can be used as a distinguishing characteristic of a seed or spore. Hilum (biology) In botany, a hilum (pronounced ) is a scar or mark left on a seed coat by the former attachment to the ovary wall or to the funiculus (which in turn attaches to the ovary wall). On a bean seed, the hilum is called the "eye". For some species of fungus, the hilum is the microscopic indentation left on a spore when it separates from the sterigma of the basidium. A hilum can also be a nucleus of a starch grain; the