The Mishna on our daf teaches the basic rule about s’khakh. The “roof” of the sukka must be made from something that grows from the ground and is in its original form – i.e. is has not been made into a serviceable item (a keli) which is subject to ritual defilement. Nevertheless, the s’khakh must be detached from the ground. If a person were to cover his sukka with a growing grapevine, gourd or kissos, the sukka could not be used, unless there was more kosher s’khakh than growing vines, or if the vines are cut.
Rabbeinu Yehonatan explains that these particular plants are mentioned because they are climbing plants that offers a lot of shade, which makes them ideal, in theory, to put on top of a sukka.
The kissos mentioned in the Mishna is a climbing plant from the family of Araliaceae. In Israel the most common form of the plant is the Hedera helix, a green ivy whose leaves are similar to grape leaves. The ivy climbs on walls, fences and trees with the assistance of grasping roots that grow from its leaves.
Rashi points out that the general principle limiting s’khakh to things that grow from the ground is not as clear as would initially appear. There are areas of halakha, for example, that consider living animals as “growing from the ground,” since their sustenance comes from eating plants.
When Rav Dimi came from Eretz Yisrael to Babylonia, he said that Rabbi Yohanan said that the verse states: “You shall prepare for you the festival of Sukkot” (Devarim 16:13). The expression “festival of Sukkot” likens sukka to the Festival peace-offering [hagiga]. Just as the Festival peace-offering is an item not susceptible to ritual impurity, and its growth is from the ground, as animals draw nourishment from vegetation, so too, the roofing of the sukka must be a substance that is not susceptible to ritual impurity and its growth is from the ground.
The Gemara on the next daf points out that this would seem to include animals as being appropriate to use as s’khakh, but Ravin quotes Rabbi Yohanan as pointing to that same passage (Devarim 16:13), which defines the holiday of Sukkot as taking place during the harvest season, which is understood to connect the sukka and the s’kakh to crops or plants.