Wood is xylem. When counting the annual rings of a tree, one is counting rings of xylem. The main components of phloem are sieve elements and companion cells . Sieve elements are so-named because their end walls are perforated. This allows cytoplasmic connections between vertically-stacked cells.
Sieve elements have no nucleus and only a sparse collection of other organelles.
They depend on the adjacent companion cells for many functions. Companion cells move sugars and amino acids into and out of the sieve elements. Water follows by osmosis . These materials then move into adjacent sieve elements by diffusion through plasmodesmata . The pressure created by osmosis drives the flow of materials through the sieve tubes.