The "citrons" that are shipped from Palestine to Ireland are one of the original citrus fruits, ancestors of modern-day lemons, limes, oranges, grapefruits, and other products of hybridization. They play an important role in the Jewish Sukkot ritual, as Bloom thinks when he remembers Moisel saying, "Must be without a flaw."
Scientists believe that the four original, purely natural citrus fruits were the citron, the pomelo, the mandarin, and the papeda. Through cross-breeding human beings have developed the forms that are more familiar today. Citrons resemble lemons, but they have a thick, leathery rind and a small, dry, unpalatable pulp. The outer part of the rind, called the flavedo, is very fragrant, as Bloom recalls happily: "Nice to hold, cool waxen fruit, hold in the hand, lift it to the nostrils and smell the perfume. Like that, heavy, sweet, wild perfume." The larger white part of the rind is edible, and is cooked with sugar to make jams, preserves, relishes, pickles, and fruitcakes.
Jews use the citron ritually during Sukkot, the week-long Feast of Tabernacles or Feast of Ingathering. This autumn holiday celebrates both the agricultural harvest and the deliverance from slavery in Egypt, when the Hebrews lived in tents for 40 years. On each day of the week, a citron fruit (the ethrog or etrog) is brought into the synagogue and waved about with three species of greenery: a closed frond of the date palm tree and leafy branches of willow and myrtle trees (collectively called the lulav, though the term also applies to the date frond alone). Various kinds of symbolism attach to these Four Species, and the specimens must be "without a flaw," as Moisel says. Relatively large sums of money can be spent on them, especially for the etrog. Bloom thinks "They fetched high prices too, Moisel told me."