The protoxylem is thus found in the central core and the metaxylem in a cylinder around it.
2.
Protoxylem occurred both at the tips of the lobes of the xylem strand and in the centre.
3.
Internal phloem is mostly primary, and begins differentiation later than the external phloem and protoxylem, though it is not without exceptions.
4.
The xylem development was'mesarch', i . e . the first maturing protoxylem had later maturing metaxylem on either side.
5.
The plant fossils found show many structural and anatomical adaptations to an aquatic environment, including a reduced vascular system, aerenchyma in tissues ( air spaces to provide buoyancy ), and protoxylem lacunae surrounded by a ring of cells with thickened inner walls.
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