The Mishna teaches that the food for an eiruv teḥumin can be placed on top of a pole or post that has been uprooted and placed in the ground, implying that it cannot be placed on a tree that is growing and rooted in the ground. The Gemara explains that this ruling follows the opinion of the Rabbis who believe that a shevut – a rabbinic decree – also applies during the bein ha-shemashot (twilight) period just before Shabbat definitely begins, which is when the eiruv takes effect.
This ruling leads the Gemara to discuss a number of cases where plants may be used on Shabbat.
The Gemara relates that a certain army [pulmosa] once came to Neharde’a and took quarters in the study hall, so that there was not enough room for the students. Rav Naḥman said to the students: Go out and create seats by compressing reeds in the marshes, and tomorrow, on, we will go and sit on them and study there.
In this case, when the army was stationed in Neharde’a, and soldiers were billeted in the yeshiva facilities, leaving little room for the students, Rav Naḥman suggested that they make chairs from the reeds near the lake so that study on Shabbat could take place there. Rami bar Hama brought our Mishna as a proof-text that use of the reeds should be forbidden while they are still rooted in the ground. Rav Naḥman responded by distinguishing between reeds that had hardened and are considered trees – which have a rabbinic decree forbidding their use on Shabbat – and soft reeds that are considered vegetables, on which no such rabbinic decree was ever established.
Rav Naḥman proves that this distinction exists by quoting two baraitot, one that includes reeds in a list with trees, like the higi and the atad, while the other lists them with vegetables like kidah and urbani.
The atad is the well-known boxthorn tree that accepts the challenge of leadership in Yotam’s parable (see Shoftim, ch. 9) after the position was turned down by the olive tree, the fig tree and the grapevine – all of the fruit-bearing trees of significance.
The atad is identified as a Lycium plant belonging to the Solanaceae family. It reaches a height of about 10 feet and grows wild in the desert. Its sharp thorns make it a prime candidate for the threat of the leader that sinks his claws into his constituency, destroying them together with himself, as represented in Yotam’s parable regarding his half-brother, Avimelekh.