Each of the lobes has a prominent central vein with 4-10 pairs of sori on either side. These sori are found about half-way between the central vein and each margin. When they are immature the sori are greenish white and later they become redish brown as seen below. Only the upper to middle lobes have sori and won’t be found near the base of the frond near the root.
Common Polypody grows from wood underground rhizomes that can be found stretching horizontally on the ground. Each frond grows along the rhizome and makes the entire group look linear.
You can see the old knobs still on the rhizome from head fronds.
Below you can see the fiddlehead.