Re: How does a plant callus cell differ from a plant cell?

Hello,
I am sorry for the delay, but i have been travelling and could not access 
my mailbox.
First, some basic concepts must be clear: The formation of a callus in not 
a differentiation process, it is the opposite (a "dedifferentiation"). Some 
plant cells (we call them meristematic cells) have the ability to become 
any other cell in a process called differentiation. It is easy to 
understand if you remember that the same process occurs in animals: the 
whole organism comes from the same fertilised female cell. The daughter 
cells have to become muscle cells, brain cells, blood cells and so on.
In plant cells, the same process occurs: a few cells in the "meristematic 
region" of the plant (usually in the tips of the roots and shoots) have to 
divide and generate daughter cells that will become different cell types 
(epidermis, parenchyma, etc.).
When you induce a callus, you are forcing these cells to reverse the 
differentiation. These cells in the callus are simply growing and 
dividing again, and are able to generate any other cell type, when properly 
induced (but that is another question).
Usually this is done with large doses of auxin (a plant hormone very 
closely related to growth) and is more likely to happen in young tissues 
(leaves, petioles and flowers are the most usual sources, but virtually 
anything can be tested as a source of fresh cells).  Of course, the cell 
outside it's usual location (the plant) and exposed to high doses of 
hormone present many metabolic modifications. That is why you found these 
differences in the calli cells. This area of plant cell culture is very 
empirical and every tissue has it's own characteristics. There is no 
absolute rule and anything can be tested. It is common, for example, get 
different results with cells from different cells in the same culture 
medium.
Hope that was what you wanted to know.
Cheers,
Marco.