Solution:

• Wood is the entirety of the secondary xylem produced by the cambium throughout the life of the tree and which accumulates till the death of the plant. Nevertheless, not the same is the faith of the secondary phloem formed outwards by the cambium.

• It does not include all the phloem elements produce during the life of the plant, for the outer, older parts continuously split, develop into the rhytidome and finally are sloughed away due to the intense dilatation growth. Thus, secondary phloem is only a thin stripe in the outer region of the woody stem containing only the youngest annual rings.