It's more a question of the length of the hairs. They can form a solid cobweb if the distance between the leaves isn't too great. On my four inch cobwebs, the centers retain a full cobweb but as the leaves expand, the cobweb breaks apart and you just get a tuft of hair at the terminus of the leaf.
The hairs are pectinaceous, which makes them sticky. In nature they are there to collect dew that is fed back into the plant. I'm sure my semps didn't need to collect any dew moisture this spring!!
Hope that helps!
Kevin
