protoxylem

Each stem contained vascular tissue with one or two strands of protoxylem.

The protoxylem was probably inside the xylem, maturing from the inside out.

The first xylem to develop is called 'protoxylem'.

In appearance protoxylem is usually distinguished by narrower vessels formed of smaller cells.

The protoxylem is thus found in the central core and the metaxylem in a cylinder around it.

Metaxylem develops after the protoxylem but before secondary xylem.

There are four main patterns to the arrangement of protoxylem and metaxylem in stems and roots.

Metaxylem has wider vessels and tracheids than protoxylem.

Functionally, protoxylem can extend: the cells are able to grow in size and develop while a stem or root is elongating.

Protoxylem occurred both at the tips of the lobes of the xylem strand and in the centre.

It includes protoxylem and metaxylem.

The stems had a central vascular column in which the protoxylem was exarch, and the metaxylem developed centripetally.

The metaxylem is thus closest to the center of the stem or root and the protoxylem closest to the periphery.

In living plants, pitted tracheids do not appear in development until the maturation of the metaxylem (following the protoxylem).

The metaxylem is thus on both the peripheral and central sides of the strand with the protoxylem between the metaxylem (possibly surrounded by it).

These ferns are characterized by root steles having 3-5 protoxylem poles and antheridia with 6-12 narrow, twisted or curved cells in their walls.

As it develops in young plants, its nature changes from protoxylem to metaxylem (i.e. from first xylem to after xylem).

The xylem development was 'mesarch', i.e. the first maturing protoxylem had later maturing metaxylem on either side.

Primary xylem is exarch upon maturation, with protoxylem strands positioned around the tips of the primary xylem arms.

Internal phloem is mostly primary, and begins differentiation later than the external phloem and protoxylem, though it is not without exceptions.

Actinosteles are typically exarch (protoxylem external to the metaxylem) and consist of several to many patches of protoxylem at the tips of the lobes of the metaxylem.

This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the protoxylem (first-formed xylem) of all living groups of plants.

Stage I: The first morphologically identifiable stage is the asymmetric division of two cells of the pericycle, termed pericycle founder cells, which are adjacent to the protoxylem poles and from which the lateral roots are derived entirely.