In answer to Harvey Chao's question, the answer is "both."  The typical
reed stop in a pipe organ is a beating reed type of pipe, but there are
some relatively few reed sounds that are produced by flue pipes.  The
study of organ reeds can be quite involved but this is the simple
answer.

Many other stops that someone not familiar with organ terminology might
be surprised to learn that Trumpet, Tuba, as well as some other pipe
sets that are labeled as what they would ordinarily consider 'brass'
instruments, are usually reed sets in a pipe organ.  Generally
speaking, the woodwind and brass sounds are produced by beating reed
pipes.

Hal Davis