The many different ways that an etrog can be invalidated for use for the mitzvah of the 4 minim - with a focus on an unripe etrog, and the dispute about it. Also, on making sure that the etrog is beautiful - with a story about R. Akiva and his super-large etrog. His partiality to this etrog seems to point to the subjectivity of beauty, yet there do seem to be some objective standards as well. Plus, binding the lulav with its own species - a disputed point by those who would use other substances (including gold rings). Note the shaky kal va-chomer here about what materials are acceptable for the sukkah, as compared to the bindings of the lulav. Also, how much beauty is established here as the natural, unprocessed world - in tension with decorations of gold and silver.