I've been wondering this for a while and have been searching and asking for days trying to figure it out, but nobody seems to have a straight answer. I thought I had struck gold when I found this thread: https://bitcointalk.org/index.php?topic=10986.0

However the responses in the thread just point to wikipedia articles and blockexplorer. I have a computer science background so I already know what a hash tree, merkle tree and merkle root are.  Just like the OP of that thread I want to know what transactions make up the leaf node/s of the merkle tree of an unsolved block. It doesn't make sense to me that an unsolved block could have any transactions.  But if it can, what are they, where do they come from?

Also, in the above referenced thread the OP mentions a block header field called txn_count that comes after the nonce.  I've never seen that before.  Is the wiki page about block header hashing incorrect or out of date?  If that thread is correct is txn_count appended after the nonce? What is the size of the data?

This is my last ditch effort before I just go grepping through the source code of one of the miners.