“The World is Prickly and Tastes Bad” is a response to the ideas put forwards in the “The World is Green” hypothesis. Murdoch et al (1966) argued that just because there is an abundance of plant life, does not mean that this is due to a top-down predator system. Instead, this theory argues that plants themselves are responsible through the development of defenses that keep herbivores from eating them all.
Check out this cool video of a plant defense:
Mimosa pudica is a plant that moves in response to any number of stimuli. Hypotheses about why this plant has developed this mechanism usually tend towards the idea that movement, and shrinking away from movement scares larger predators, and helps shake off any bugs that may be crawling on them.
Thorns, spines and prickles are actually three different types of plant defenses, although, in the end, they all do about the same thing. From Wikipedia: “Thorns are modified branches or stems, spines are modified leaves, and prickles are needle-like extensions of the cortex and epidermis” (http://en.wikipedia.org/wiki/Thorns,_spines,_and_prickles).
Acacia trees are a great example of a tree with many defenses. Besides thorns of both the straight and the hooked variety, acacia trees also produce a toxic tannin, and have developed a symbiotic relationship with ants.
So all in all, plants can be pretty surprising!