Over the summer months Mavis Eggwhistle has hinted in no uncertain terms that Bart should increase his level of geniality and hold a harvest picnic for the Farmhands and neighbours. In an effort to convince Mavis that he was a bona fide exponent of the art of geniality, he set to and did some research on his family tree.
As part of this work, Bart has grown a giant Broccoli Tree, which you can see here displayed on that part of his farm he refers to as Dinner Plain Flats. Bart claims, perhaps correctly, that this Broccoli Tree, leaving aside its terrible woody core, would victualise any number of picnicners. An argument with Mavis ensued over the words, geniality and geneology , wherein Mavis may or may not have walloped Bart with the 1962 Edition of the Websters American Dictionary, so, as Mavis says, Bart can feel the positive effects of book learning.