Formal language theory is by its very essence an interdisciplinary area of
science: the need for a formal grammatical or machine description of specific
languages arises in various scientific disciplines. Therefore, influences from outside
the mathematical theory itself have often enriched the theory of formal languages.

Perhaps the most prominent example of such an outside stimulation is provided
by the theory of L systems. L systems were originated by Aristid Lindenmayer
in connection with biological considerations in 1968. Two main novel
features brought about by the theory of L systems from its very beginning are (i)
parallelism in the rewriting process-due originally to the fact that languages
were applied to model biological development in which parts of the developing
organism change simultaneously, and (ii) the notion of a grammar conceived as a
description of a dynamic process (taking place in time), rather than a static one.
The latter feature initiated an intensive study of sequences (in contrast to sets) of
words, as well as of grammars without nonterminal letters. The results obtained
in the very vigorous initial period-up to 1974-were covered in the monograph
"Developmental Systems and Languages" by G. Herman and G. Rozenberg
( North-Holland, 1975).