Hi Michael -
Yes, I know it seems weird until you understand the reasons why. Much has
been written on this topic and someone on this list already offered a
reprint of his own article on the subject.
But to summarize:
Plant chimeras generally result from a mutation in only a portion of the
plant's tissue. This mutation can occur in an entire layer of tissue
(Periclinal), or only a portion of a layer (Mericlinal) or only a portion
of several layers (Sectorial). When it occurs in an entire layer, the
chimera tends to be more stable. In the other two cases, it is unstable -
this is due to where a shoot originates on the stem. If it originates from
an area that is without the mutation, it grows normally; if it originates
from an area with mixed mutant and normal tissue, it will also be mixed; if
it originates from an area of only mutant tissue, it will contain only
mutant tissue. leaf or floral variegation is the most obvious type of
chimera, but others do exist (Harry Lauder's Walking Stick (Corylus
Avellana 'Contorta') is a mix of slow growing and regular tissue, which is
responsible for the contorted growth of this famous plant).
An example of the unstable can be seen in:
http://www.glasshouseworks.com/images/plecvrtv.jpg (Plectranthus verticillatus)
Depending upon where you take of cutting from such a plant, you may get
very different looking plants (variegated a little, a lot, or
non-variegated). Unless one of these shoots happens to produce a
Periclinal chimera (it can happen), the variegation will continue to be
random and unstable.
An example of a stable chimera can be seen in:
In this common variegate, there is a lack of chlorophyll in the tissue
layer that grows into the leaf edge. I have a 'sport' of this plant in
which the mutation 'switched' into a different layer, creating a plant very
much like this one:
http://www.glasshouseworks.com/images4/plect-lothlorien.jpg (Plec. mad.
'Lothlorien'), which has a green edge and a pale central area. The stems
are also pale because they originate from the same tissue as the central
leaf area. Both of these variegates are relatively stable, but each can
produce odd shoots from time to time.
I hope this helps a bit - it is a complicated but fascinating topic.
h o r t u l u s a p t u s - 'a garden suited to its purpose'
Seán A. O'Hara firstname.lastname@example.org www.hortulusaptus.com
1034A Virginia Street, Berkeley, California 94710-1853, U.S.A.
(ask me about the worldwide Mediterranean gardening discussion group)