Blackberry plants, like the one pictured above, are known for their painful and annoying thorns. These thorns are part of the stems of the blackberry bush, and are a modification of the stem for defense. Nobody wants to eat a plant with a mouthful of thorns, right?
Tendrils are grown and used by parasitic, invasive and/or climbing plants to spread and provide anchor points for further growth. In some species, tendrils will curl around anchor points and other plants, called hosts, but interestingly never curl around themselves. Pictured above is the end of a long 15 foot blackberry tendril pulled out of a tree in the grand forest.
List #106
Patrick Ledoux's AP Biology Blog 