The Bitter Truth

Does Romaine lettuce have anything to do with detergent, stoning, and putting on pants? Lettuce be clear: the Shulchan Aruch says they do.
There is a question regarding whether we can derive halachos from the order of items in a Mishna. Rav Huna states that generally one may not, although there are exceptions. For instance, the Mishna lists a series of chemicals that can be applied to blood to render it no longer tamei (according to Rabbeinu Tam). Rav Huna maintains that these chemicals must be applied in a precise order, otherwise it would be a washout. Similarly, our Mishna in Sanhedrin (49b) lists the punishments in a specific order, starting with the most severe, stoning. However, when the Mishna enumerates the garments that a Kohen wears, the order is not precise. The Mishna mentions the shirt first, even though the Kohen should put on his pants first.
The Mishna in Pesachim (39a) lists five vegetables that can be used for marror: chazeres, ulshin, tamcha, charchavina, and maror. The first vegetable is identified as lettuce, while the second is identified as endives. Some root for interpreting the third vegetable as horseradish. Is there a preference for using lettuce over endives? The answer should be no. The Mishna in Pesachim is not included in Sanhedrin among the exceptions, and therefore, Rav Huna’s dictum applies: the order of the Mishna is inconsequential and not worth losing your chrain over.