Reeds and Bellows
Most harmoniums have two sets of reeds for each key, tuned an octave apart. The lowest set of reeds are called the bass reeds, and the higher reeds are called male reeds. Some larger harmoniums have a third set which are an octave higher than the male reeds, called, not surprisingly, female reeds, but these are not often used for kirtan.
The reeds are mounted on the bottom side of the key/reed board.
Pressing a key releases this air across the reed(s).
The air flows into the body of the harmonium and into a triangular manifold which directs the air into the bottom of the case where the second bellows is. Springs on the bottom of case push up on the lower bellows forcing air through any open stops and into the reed chamber, where pressing a key causes the reed or reeds to vibrate, making beautiful music.
Bottom-bellows springs in the bottom of an upside down harmonium with the bottom cover removed.
Disassembled harmonium with key/reed board titled back
Disassembled harmonium with reeds exposed
Reeds are identified by note name and a number for the octave. The lowest bank of reeds on Delhi harmoniums are called the Bass reeds. The lowest C bass reed is a C3, and they go up from there. The next C would be C4 and so forth. The second bank of reeds is called the male reeds and are one octave above the bass reeds. The stops on the front of the instrument control the air flow to each bank.
On Calcutta harmoniums, the lowest reed (excluding the lower reeds on a scale changer harmonium) is a C2.
If you need to order a replacement reed, it is important that you get the correct note name and octave. This Wikipedia page describes everything you would ever want to know about pitch names.
My recommended source for replacement reeds is Old Delhi Music.
In fact I recommend Old Delhi for all things harmonium related.