Following a discussion of oils that are appropriate for use in lighting Shabbat candles, the Mishna on today’s daf turns its attention to the wicks that are used. Common wicks such as cotton are made from seeds; the Mishna discusses the fact that most substances derived from actual trees would not be appropriate for use as wicks.
Of all substances that emerge from the tree, one may light only with flax on Shabbat because the other substances do not burn well (Tosafot). And of all substances that emerge from the tree, the only substance that becomes ritually impure with impurity transmitted by tents over a corpse is flax. If there is a dead body inside a house or a tent that is made from any materials that originate from a tree, everything in the house becomes ritually impure. However, only in the case of flax does the tent itself become impure.
Cultivated flax, Linum usitatissimum, is an annual plant that grows erect to a height of 40–120 cm. Its flowers are blue or white. Its stiff stalks contain flax fibers, and oil is extracted from its seeds. After the plant is cut, the stalks are soaked in water, called mei mishra in the language of the Sages, for several days. Various bacteria cause the materials that attach the fibers to the stalks to decompose. Afterward the shell is beaten and opened and the fibers are extracted to be used in weaving linen, bad or shesh in the language of the Torah. The flax plant has been cultivated since ancient times, especially in ancient Egypt.
With regard to the secondary halakha that is presented in the Mishna, the only material made from plant fibers that is suspended over a dead body that becomes ritually impure is linen. Some commentaries say that this law applies specifically to a permanent tent (Rambam Sefer Tahara, Hilkhot Tumat Met 5:12).