Re: where are the nuclei positioned and why?

If you are observing onion, elodea and mammalian cells under a microscope,
you will observe the nucleus (singular) as a large dark, round spot inside
the cell.    The nucleus is the center for DNA storage, replication and
transcription into RNA.  Within the nucleus, depending upon the resolution
of your microscope and the quality of the staining, you may be able to see
from one to several "nucleoli," (plural) which present as dark spots within
the boundaries of the nucleus itself. The nucleolus (singular) is the name
we give to this area of the nucleus where ribosomes are assembled, but it
is not a separate organelle.  It's really more like an aggregation of
molecules within the nucleus and it shows up as darkened spots under the
microscope.  For some great shots of onion nuclei and nucleoli, visit
www.microscopy-uk.org.uk/mag/artnov03/wdonion.html.

When you think about the location of the nucleus within the cell, the
nucleus is usually somewhat centrally located, but not always.  When you
look at cells through a microscope, you must keep in mind that you are
observing a two-dimensional cross-section of the three-dimensional cell. 
The microscope can show you the nucleus' position within the x and y
planes, but you do not see its relative position in the z plane.  This may
contribute to why the nuclei appear to be centrally located when, in
actuality, they are not.  In addition, we get a skewed view of cells by
looking at fixed cells.  Remember, cells are alive, and you must get used
to thinking of live cells as more of a "fluid mosaic" model.  Keep in mind
that the relative position of the organelles is dynamic -- always changing
-- not fixed.  

Good question . . . hope that helps, and remember to keep your perspective
when looking at cells under the microscope!