Muggles' Guide to Harry Potter/Magic/Mimbulus Mimbletonia

Overview
Mimbulus Mimbletonia is a spineless, cactus-like plant with a unique defence mechanism.

Extended Description
Our introduction to this plant occurs when Neville displays it on the Hogwarts Express in Harry's fifth year. Demonstrating its special features, Neville deliberately triggers its defence mechanism, completely covering Harry, Ginny, Luna, and himself in stinksap, mere moments before Cho Chang looks in to say "hello" to Harry.

Analysis
We are told that this plant is quite rare and valuable, and can gather from this that it is very hard to care for. At the end of the book, as we return to the Hogwarts Express for the return home, we see that it has thrived and grown while at the school. We will learn, at the beginning of Harry Potter and the Half-Blood Prince, that Neville did quite well on his Herbology OWL, gaining an Outstanding grade; the fact that this plant has done so well reinforces the feeling we have that Neville, for all his shortcomings and self-doubts, is quite skilled at taking care of magical plants.

The purpose of this plant seems to be to increase Neville's self-esteem while damaging Harry's. Neville's obvious liking for this plant and his ability to care for what we are told is a rare and difficult species are rewarded both by the plant's thriving and by Professor Sprout's comments. Neville is greatly heartened by the recognition that he is good at something – his career at Hogwarts seems, in his own eyes, to have been largely devoid of successes. Harry, meanwhile, is in the earliest stages of a relationship with Cho Chang, and feels that he must make a good impression; he feels, correctly, that his image in her eyes is somewhat dimmed, at least temporarily, by the fresh coat of stinksap he has received at Neville's bidding.