At 12:14 PM 10/30/96 -0800, Dr. David Starrett wrote:
>Plant-eders,
> While teaching plant anatomy, I came across the tidbit that xylem mother
>cells divide twice while phloem mother cells divide once. When I brought
>this up to the class, one student asked whether that one division was the
>division producing the Companion cell. I tried looking this up in a few
>texts, but found no answer. Anyone have an idea whether the single PMC
>division yields the Sieve Tube Memebr plus companion cell?
>> Any thoughts would be appreciated!
>I'm unsure where you would have picked up that "tidbit" David, but the
impresion it gives me (and I suspect your students) is that the cambial
initial divides and gives rise immmediately to either a xylem or phloem cell.
Even with high resolution electron microscopy it is impossible to
distinguish a cell as *the* cambial initial. That's why the cambium is best
referred to as a zone, rather than a singel cell layer. In fact, there are
multiple periclinal divisions in the cambial zone. If the cells on the
phloem (external)side of the cambium are destined to differentiate into
sieve tube members (we have to restrict the discussion here to angiosperms),
a subsequent periclinal division within the cell will "cut off" the
companion cell. Other periclinal divisions of the cambila initials would
result in the production of phloem parenchyma cells.
If we were considering a gymnopsperm with albuminous cells (functionally
equivalent to a companion cell, but not of immediate ontogenetic derivation
from the sieve element, perilinal divisions within the cambial zone would
give rise to sieve cells, albuminous cells and phloem parenchyma cells.
I would be curious to know the source of the "tidbit." I would bet it was
not Raven, Evert amd Eichhorn, Esau or Fahn. Inasmuch as there is typically
many more cells produced on the xylem side of the cambium, it's not
surprising that the "two divisions" was mentioned, but that in itself seems
less than accurate.
James W. Perry, Ph.D.
Campus Dean/Associate Professor of Biological Sciences
University of Wisconsin - Fox Valley
1478 Midway Road, P.O. Box 8002
Menasha, Wisconsin 54952-8002
office:(414) 832-2610
home: (414) 836-9959
FAX: (414) 832-2674
e-mail: jperry at uwc.e
