A stolen willow twig or a dry one is invalid.  [One] from an asherah or from a city that was led astray is invalid.  If its top was broken off, [or] its leaves were severed, or [it is] a tzaftzafah, it is invalid.  [One whose leaves are] wilted, or part of its leaves have fallen off, or [one] from a field, is valid.

R’ Yishmael says: Three myrtle twigs, two willow twigs, one lulav, and one esrog [are required]; even if two are broken off and one is not broken off. R’ Tarfon says: Even if all three are broken off.  R’ Akiva says: Just as there is one lulav and one esrog, so [there is] one myrtle twig and one willow twig.