In the burr plant, which one that stuck on the fur of animal seed or fruit

It is the fruit that gets stuck to the animal fur .

Click on the link below to see burr on a fruit - the seeds are inside -

http://www.cottoncrc.org.au/files/1a6538…
http://www.cottoncrc.org.au/content/Indu…

On the link below there are a number of burr fruits with a SMOOTH WALLED seed inside
http://www.google.co.in/search?q=burr%20…