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HYB:Cells and pigments

On Iris-photos Linda Mann speculates about the relative placement of
carotenoids in plastids as compared to the location of the vacuole where the
majority of the anthocyanins are present.

Let me describe again the structure of the cell.

First of all, the cell, either plant or animal, is a flexible bag.  The skin
is the cell membrane, and is quite strong, but permeable to specific things in
specific ways.

Plants differ from animals by having <also> outside the membrane a wall of
relatively inflexible material.  Thus, plant cells end up a 3-d version of a
hexagon or something similar to it.  Epidermal cells, those with which we are
concerned, as they carry the great majority of the pigments in the petal, have
all sorts of different shapes on the exposed surface--which give the
impression to our eyes of <texture>.  Varying textures mean different cell
surface shapes.

These varying shapes affect also how we perceive the colors.

Now--plastids are in the cytoplasm--that bunch of stuff with a lot of stucture
and organization that is outside the nucleus.  Just where in the cytoplasm
plastids are found depends on how the vacuole and nucleus push the rest of the
cell contents against the surrounding 3-d cell membrane and wall.

Plastids are scattered in the part of the cytoplasm closest to the membrane
and wall.  Since an essential part of their work is the capturing of certain
wave lengths of light and storing the energy, later yielding that energy to
other mechanisms in the cell, they have to be where those wavelengths reach
them.  The upper surface is rich in plastids.  If any are present, and I
believe I remember they are,--are present on the opposite side of the cell in
the cytoplasm there, they will likely be fewer by far.  Only a microscope
study or an illustration by one who has studied cell structure could say just
how this array occurs.

Looking into the cell from its surface the plastids are encountered first.
Beyond them is the vacuole where the anthocyanins are seen (and the AVI
structure if present, which allows a far higher concentration of anthocyanins
than the normal limit).

Since the anthocyanins are dispersed like water colors in a glass of water,
they are seen through the space between plastids.

But light is travelling both directions through the cell.  The supporting
layer of parenchymal cells is more translucent than transparent (in other
words, appears white), but there is an epidermis on the OTHER side of the
petal, which may have an entirely different distribution of colors in the
plastids, or in the vacuoles.

The net result is--by the light refracted and sent back to your eye, you are
seeing photons that have bounced back from plastids, photons which have
penetrated to the vacuole "bag"-like area and reflected back, some of which
are trapped and absorbed by the plastids, and perhaps some photons which have
entered the cell  from the other side of the petal, which have passed through
two epidermal layers containing plastids, and may or may not have passed
though either or both vacuoles.

What you see is a very complex mix of wavelengths, no matter what net color
effect you observe.  A spectral analysis of the light reaching your eye from
any iris bloom is not simple.

Or did you want a simple answer?  Can't be done.  Simply not simple.

Neil Mogensen  z 7 western NC mountains

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