Studies (1—4) have shown that it is possible to distinguish different
stages of phloem tissue differentiation in the developing roots of Lemna
minor by examination in the transmission, scanning, and optical microscopes.
A disorganized meristem, immediately behind the root-cap, gives rise to the
vascular tissue, which consists of single central xylem element surrounded
by a ring of phloem parenchyma cells. This ring of cells is first seen at
the 4-5 cell stage, but increases to as many as 11 cells by repeated radial
anticlinal divisions. At some point, usually at or shortly after the 8 cell
stage, two phloem parenchyma cells located opposite each other on the ring
of cells, undergo an unsynchronized, periclinal division to give rise to the
sieve element and companion cell. Because of the limited number of cells
involved, this developmental sequence offers a relatively simple system in
which some of the factors underlying cell division and differentiation may
be investigated, including the distribution of diffusible low atomic weight
elements within individual cells of the phloem tissue.
