Each strand of spiral or annular first-formed tracheids is called a protoxylem strand, as distinct from the metaxylem or rest of the xylem, which consists of thick-walled tracheids, the pits of which are often scalariform.
The protoxylem of each is a leaftrace, while the metaxylem consisting of a right and a left portion forms a quite distinct cauline system.
Sometimes, however, the centre of a bulky root stale has strands of metaxylem (to which may be added strands of metaphioem) scattered through it, the interstices being filled with conjunctive.
The differentiation of metaxylem follows according to the type of root-stele, and, finally, any stereom there may be is developed.