What material continuously splits due to secondary growth in trees?

The material that continuously splits due to secondary growth in trees is the outer bark. As trees undergo secondary growth, which is the process by which they increase in girth, they produce new layers of secondary phloem and secondary xylem. The outer bark, or periderm, is made up of several layers including the cork cambium that generates the cork, which serves as the actual protective outer layer.

As the tree ages and grows, the rate of secondary growth can create pressure on the outer layers. This increased growth causes the outer bark to stretch and can lead to splitting, especially since the outer bark is composed of dead cells that do not expand as readily as living cells. This is a natural occurrence in many tree species and is indicative of their ongoing growth and life processes.

While the cork cambium does indeed play a role in forming part of the bark, it is the outer bark that is most directly associated with the splits observed during tree growth. The inner bark, which includes the phloem, typically does not split in the same way, as it consists of living cells that are more flexible. Pith rays are involved in storage and transport but do not experience significant splitting related to secondary growth.