Google kindly found lots of photomicrographs for me when I searched for sections through stinging nettle stems, of which one or two actually were what I was looking for. Sciencephoto has the image that most closely resembles the distinctly cruciform stems of the nettles we’ve collected. I’ve redrawn it here, simplified:
From the Sciencephoto caption and my close inspection of nettle stems, I think the spinnable fibres/phloem tubes are the four stripes of turquoise in the bulges: when I peel the nettle stems, pulling the fibre + other tissues off the central pith, the material I peel separates into four strips running on the bulges separated by thinner areas with no visible fibres from within the hollows.
Here is a view of nettle stems taken from my impromptu retting tank. The hollow between two bulges can be seen on the top of the stem held between my fingers and thumb.
And this is a section of the ‘skin’ peeled from a nettle stem after much of the unwanted plant tissue was scraped away from both sides of the fibres. The ‘skin’ has separated into four strips as described above; note that the tissue on either side of the gaps appears almost translucent, with no white fibres visible.
As the goal of fibre processing is to maximize the fibre yield from each stem, it seems possible or even likely that traditional users of nettle fibre would have learned which plants reliably supplied more fibre, perhaps selecting and cultivating stems that did not have the fibre-free hollows.