I should clarify, since some of the research papers on the nicotine-in-stems issue are unclear. Tobacco stalks are sometimes identified as "stems" in studies. [Boo!] When I refer to stems, I am speaking of the central vein of each leaf. It is this central vein that is milled and used for commercial snus and commercial chew, and is expanded, flattened and shredded for cigarette filler.
Quantitatively, the difference in nicotine concentration between central vein and leaf lamina is quite variable. (Even just within the lamina, the nicotine varies from one portion of a leaf to another.) But it general, for any given variety and priming level, the average nicotine of the central vein is lower than the average nicotine of its leaf lamina.
Some years ago, I saved the cured stems of every leaf, bagged by separate varieties. If finely minced, the stems (central veins) can be packed into a pipe and smoked. The burn is okay, though it is a little fussy to pack. Aroma and nicotine are timid. Stems can also serve as the center of a twist, so that coins cut from the finished twist have a "birdseye" in the center.
[CODA: I think I still have those bags of 8 year old stems somewhere, but I routinely discard stems.]
Bob