Xylem also contains tracheids . Like xylem vessels, they have thick, lignified walls and, at maturity, no cytoplasm. Their walls are perforated so that water can flow from one tracheid to the next. The xylem of ferns and conifers contains only tracheids. In woody plants, the older xylem ceases to participate in water transport and simply serves to give strength to the trunk.
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.