They are root primordium.

Al

Here is a link that might be useful: Check me out.

Al,

How come many of these don't seem to produce roots, they simply turn brown and callous, like bark?

Root initials (remember, we cannot see initials. They are hidden beneath the cortex tissues) organize, form a root cap, and push through bark tissues (primordia, at this stage). If they are in close proximity to vascular bundles, they make a connection and continue developing. If they are not, the plant absorbs any mobile nutrients contained in the primordia and connecting tissue and translocates it for use in root formation or other organs.

Al

Thank you so much for explaining the white fluffy stuff!! I've wondered that for a long time. Now I know its primordium!

Now, if it would just be possible to figure out how to tell which will strike roots and which will not, we will be in business ;)

Probably the following may shed some light pictorially (pick and paste):
http://mycor.nancy.inra.fr/IMGC/PoplarGenome/PoplarDB/FEBS/Rooting_stages.pdf

The slide at left top shows the stuff Paully mentioned and which we observe very often mostly in baggie method when cuttings are wrapped in paper or when cuttings are burried in sand.
Also, the very last slide (right bottom) shows many of such white stuff resulting in root formation. I have never personally observed such spots resulting in roots but probably it may need time and patience for it to happen if it ever happens.

Sorry - I may have misunderstood. The box in the upper left hand corner of the link you shows a 'lenticel', which is a structure that facilitates gas movement (exchange) in and out of the twigs & branches.

Here is a picture of lenticels on a mulberry twig and some more info. Mulberry (Morus) is also in the Moraceae family (like all species/cultivars of figs) and a very close relative of Ficus c.

Al

At the point of the lenticel is where we see the primordium develop...correct?

-Little John

Primordium usually develop from root initials in easy to root species like carica, but de novo roots often form @ any of several other locations too, lenticels being only one of them.

Not only is it necessary to keep moisture levels low enough that a film of water doesn't impede O2 flow into the cutting via the proximal end, it's also bad form to have media so moist that lenticels are clogged with water.

Al