It is useful to divide agricultural fields for a variety of different reasons. Traditionally, fields are not separated only by fences, but also by other types of barriers.
The Gemara on our daf discusses Rabbi Yoḥanan’s approach to two types of divisions – metzer (a boundary) and ḥatzav (sea squill, a type of plant planted in order to separate between fields). According to Rav Asi, Rabbi Yoḥanan believes that simple separations like these would divide a field in the case of nikhsei ha-ger (when someone claims the field of a convert who passed away with no living heirs – see yesterday’s daf) but not in cases of pe’a (leaving a corner of the produce of one’s field for the poor) or tum’a (if someone walked through fields and is not sure whether he entered a field where there was an issue of ritual defilement, like a dead body, and became tameh or not). When Ravin came from Israel, he quoted Rabbi Yoḥanan as ruling that metzer and ḥatzav would act as a separation between fields even for pe’a and tum’a.
The point of disagreement is, apparently, based on whether these types of separations that are neither natural boundaries (like a river) nor are they clearly made for the purpose of dividing the fields (like a fence), should be recognized as an official separation. According to Rav Asi, we will accept it as such only in cases that are of Rabbinic concern – e.g. taking possession of nikhsei ha-ger. In cases where the concern is Biblical – e.g. pe’a and tum’a – we cannot accept these as divisions. Ravin understands that metzer and ḥatzav are acceptable boundaries in all cases.
Ḥatzav, Urginea maritima or sea squill, is a plant with a large bulb, which produces an abundance of green leaves during the winter months and dries up in the summer. At the end of every summer the ḥatzav sprouts tall stalks topped by small, white flowers. Both the leaves and the bulb contain poisons, which keep animals from eating them. The dried bulbs were traditionally used in the production of medicines. The ḥatzav has a long root system. Because of its sturdiness and its ability to grow again every year, during ancient times it was popularly used to demarcate the boundaries of fields, a role that it still plays in some Arab villages in the modern era.