Something that's been unclear to me from the presentations I've seen of interacting string theory: In a setting like bosonic string theory, where the interactions consist of strings splitting and joining, is it always possible for a string to split? In other words, are there "minimum-sized" strings that cannot be split further, but can only interact by joining? Or is it always possible for every string to enter into either type of interaction? (For purposes of the question, let's say it's "possible for a string to split" if there's any non-zero amplitude at all for a split, even if splitting is extremely unlikely.) Edit to add: References would be GREATLY appreciated!