There is no big difference. The cells are the same. The cells are organized the same. They both serve the same function. I don't even think there's a huge physiological difference between the roots exposed to the air and those that are in the pot. It's just that one is more adapted to drier conditions. The other is more adapt to wetter conditions.
I haven't looked at a cross section of an orchid's roots when they are exposed to air as opposed to when they are in wetter conditions, but I'm assuming the difference is in the thickness of the velamen layer or the epidermis layer, and there might even be a difference in the thicknesses of the cortical layer.
So, in other words...
The roots exposed to air might have either a thicker velamen layer or a thicker epidermis layer, and a thinner cortical layer.
The roots in the pot that are exposed to wetter conditions might have a thinner velamen layer or a thinner epidermis layer, and a thicker cortical layer.
In some cases, the difference in the diameter of the roots may also be apparent, in that the roots exposed to the air may sometimes be smaller in diameter in comparison to roots in the pot.
Plants are what are considered in the world of botany "plastic", and are therefore very adaptable. Try looking up the term, "plastic", in the context of botany.
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Philip
Last edited by King_of_orchid_growing:); 05-07-2013 at 02:02 PM..
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