Wood is a porous and fibrous material found in trees and woody plants that provides structure and transports water and nutrients. It is classified in several ways, including by the amount of parenchyma tissue (manoxylic wood contains more parenchyma and is softer, while pycnoxylic wood contains less and is harder). Parenchyma distribution is also used - apotracheal parenchyma is scattered, paratracheal occurs in masses around vessels. Wood can also be classified as non-porous or porous depending on the presence of vessels, and porous wood can be ring porous or diffuse porous based on vessel arrangement. Annual growth rings form due to seasonal cambium activity and indicate a tree's