Nick Maclaren wrote:

No, they tip in. Blackberries do not normally root except at
the tips, but the shoot may then grow on from where it tipped
in, and do it again.



All part of the colonisation process that you can see going on around
neglected fields - blackberries in hedge tip-root, thorny new growth keeps
off grazing animals and human traffic, other plant species take advantage of
the protected environment and hey presto you have a 2m wide strip of long
grass, wild flowers and sapling that would other wise have been grazed off.
As the blackberries grow on the process starts again.

Pk