As with all of the arba minim – the four species that are taken on the Sukkot holiday, the etrog is hinted at in the pasuk that refers to it as a peri etz hadar – the fruit of a beautiful tree – but is not clearly identified. The Gemara attempts to derive the identity of the fruit from the pasuk itself. This methodology is not limited only to our Gemara – the Jerusalem Talmud argues that it must be a beautiful fruit from a beautiful tree, as opposed to a beautiful tree with ugly fruit (like a carob) or an ugly tree with beautiful fruit (like a pomegranate).
Our Gemara also notes that the pasuk emphasizes both the fruit and the tree, and suggests that we are to understand that it is a reference to a tree whose fruit has the same taste as the tree itself. This apparently points specifically to an etrog, where most of the fruit is the peel – whose taste is similar to the tree – and only a very small amount of it is truly fruit.
The Gemara’s objection to this suggestion is that other types of fruit fall into this category, as well. Pepper, for example, has the same taste as its tree. The black pepper – piper nigrum – grows on a climbing vine to a height of 5 – 7 meters. Its growth is similar to that of a grape vine, as it spreads out on the ground if it has nowhere to climb. At the edges of the branches there are white sprouts, from which the fruit grows, each one about the size of a pea. When ripe, they turn red. Pepper is native to Indonesia and the southern part of India, but already in Talmudic times it was successfully cultivated in Israel.
The Ritva points out that the discussion in the Gemara about how to define the passage commanding us to take a peri etz hadar cannot possibly be searching for the true identification of the fruit. By the time of the Gemara it is obvious that there were already long-standing oral traditions that the fruit that had to be taken was an etrog. Our Gemara is simply an attempt to investigate whether the well known tradition could be shown to have a source in the written Torah, as well.