Cork is composed of suberin, a hydrophobic substance. It is related to cutin and suberin. Phlobaphen also occurs in the polyaromatic part of the suberin mixture. There are other biopolymers such as rubber, suberin, melanin, and lignin. The tree forms a thick, rugged bark containing high levels of suberin. Other examples of biopolymers include rubber, suberin, melanin and lignin. The chemistry of the Casparian strip has been described as composed of suberin. The exact qualitative and quantitative composition of suberin monomers varies in different species. The autofluorescence of polyphenols can also be used, especially for localisation of lignin and suberin. With C18&colon;1 and C18&colon;2, their content amount from 24 to 45% of whole suberin. Suberin can also be found in various other plant structures. Suberin is highly hydrophobic and a somewhat 'rubbery' material. Suberin consists of two domains, a polyaromatic and a polyaliphatic domain. Suberin is found in the phellem layer of the periderm (or cork). The Casparian strip in the endodermis roots and cork cells of plant bark contain suberin.