The cells in the lighter coloured part of the growth ring have relatively thin cell walls and large lumina (the void in the centre of the cell) and are called early wood or spring wood. Conversely, the darker wood, known as late wood or summer wood, has thick walls and relatively small lumina. Hardwoods can be classified into ringporous, semi-ring porous and diffuse porous. In the first two groups, the vessels tend to have larger diameters and/or be more numerous in the early wood compared to the late wood, so the growth rings are generally clearly defined. The growth rings of diffuse porous hardwoods are sometimes more difficult to see because the vessel diameter does not really change. However, the end of the growing season can be marked by a ring of late wood tracheids and fibres, creating a thin line.