Sukkah, Chapter 1, Mishnah 5. In the Mishnah before us, the Mishnah presents another example of something that is invalid for use as sechach.
The text of the Mishnah:
"Chavilei kash v'chavilei etzim v'chavilei zeradin - ein mesachechin bahen" - bundles of straw, bundles of wood and bundles of shoots - one may not use them as sechach. There are three types of bundles here:
"Chavilei kash" - bundles of straw.
"Chavilei etzim" - bundles of wood.
"Chavilei zeradin" - bundles of soft, tender tree branches.
None of these three types may be used as sechach so long as they remain tied together in their bundles.
The reason for the disqualification:
The Gemara explains that in and of themselves, these materials belong to the category of items that are valid for sechach: they are detached from the ground, they grow from the ground, and they do not receive tumah - by rights they should be perfectly acceptable. The concern, however, stems from the fact that they are bound into bundles: a person returning from the field with these bundles in hand may lay them on top of the sukkah in order to rest, or in order to dry them out, and afterward he will say: "This is a fine sukkah, let us leave them right here."
Such placement is invalid because of the rule of 'ta'aseh v'lo min he'asuy', which we already mentioned in the previous Mishnah: the bundles were not placed there for the sake of shade, which is the very essence of a sukkah, but only for the purpose of drying. Placement that was not for the sake of the sukkah requires that one lift the sechach and set it down again for the sake of the sukkah.
Because of this concern, the Sages decreed that even someone who placed the bundles there for the sake of the sukkah from the outset - the very fact that they are tied into a bundle disqualifies the sechach.
"V'chulan shehitiran - kesheiros": - and all of them, if he untied them, are valid.
In all of these cases, if he untied the bundle and spread out its contents, he has thereby performed an act upon the body of the sechach for the sake of sechach - and the sukkah is valid.
"V'chulan kesheiros lid'fanos": - and all of them are valid for the walls.
The Mishnah concludes with a law that applies to every category of invalid sechach: all those items that are not valid for use as sechach are valid for use as the walls of the sukkah. The criteria required for sechach are not the same as those required for the walls, and the walls may be made of almost any material.
In summary: In this Mishnah we learned that bundles of straw, wood and shoots are invalid for sechach even though in and of themselves they are suitable for it, because of the decree of the Sages lest a person place them on the sukkah to dry and then leave them there, so that his sechach turns out to have been placed not for the sake of shade. Once he unties the bundles and spreads them out - they are valid. And finally it is established that everything that is invalid for sechach is valid for use as the walls of the sukkah.