st.statistics - Finding a representative of a branch in hierarchical clustering

it really depends on how you're doing the hierarchical clustering, but if (for example) you're using a sum-of-squares measure, then the centroidal merging that Mikael recommends is in fact the exact representative.

There are entire families of clustering algorithms that rely on the ability to store representatives of lower parts of the clustering and merge them. While these are not hierarchical per se, they use a hierarchical approach, and so the concept is the same. For reference, check out the BIRCH algorithm:

http://en.wikipedia.org/wiki/BIRCH_%28data_clustering%29