I don't have a scientific reason, I suppose it is the same as why some leaf axils produce branches on trees and some
do not ;-) Nice Opuntia though.
Dan
Lincoln, NE USDA 5

Well, on cacti the aeroles are the places from which leaves, flowers and stems arise.
Of course the leaves are replaced with spines, tho Pereskias and some opuntias do have leaves too, those of Opuntia (leaf primordia)dropping off soon after emerging.

I'm admiring a great Macro pic.

Wow.

Al

Thank you all for your kind follow-up. Do you know the species of this Opuntia? I guess O. subulata, monstruose form, not sure though.
Here is another view:
{{gwi:465971}}
Davide

Dont know the species, but it looks a few years old, has it flowered?

No Larry, not yet!
Davide