Actually that's two sets of differences. Within the Jovibarba subsection any of the rollers can be crossed to the heuffs. Both of these species are diploids and the hybrids between them are sterile or nearly so.
The species in the Jovibarba section can be crossed with the other semps, but with great difficulty. The hybrids produced are abnormal and sterile.
Among the Sempervivum section, diploids crossed with other diploids give close to sterile hybrids. My Greenwich Time is an example, a montanum X calcareum cross.
The tufted group are crosses of cobwebs with other non-cobwebbed semps. They can be of three different ploidies. Diploid if both parents are diploid (and sterile), triploids if one parent is diploid and the other tetraploid (these show surprising fertility), and tetraploid (if the tetraploid cobwebs are crossed to tetraploid species and cultivars). The last group is almost fully fertile.
There will be a full discussion of this in my book!
Kevin
