The letter A indicates . A cross-section of an adult tree trunk. The outer layer interacts with the environment. The second layer is labeled C. The next layer is labeled B, and the layer after this is labeled A. The letter A indicates . A cross-section of an adult tree trunk. The outer layer interacts with the environment. The second layer is labeled C. The next layer is labeled B, and the layer after this is labeled A. apical meristem secondary xylem vascular cambium cork cambium lateral meristem

secondary xylem

Explanation:

Secondary xylem is a type of xylem that grows or derives from vascular cambium. Xylem tissue is used to transports water and minerals from root to leaf in the plants.  Such plants include terrestrial plants. Secondary xylem is basically one of the growth phases of the xylem and is produced as a result of the secondary growth of vascular cambium. Its cells involve tracheids, fibers parenchyma and has short and wide vessels and tracheids. Secondary xylem is very rich in fibers.  

the backbone of the dna and rna compound is sugar phosphate.