Esperanto parts of speech in functional discourse grammar

Esperanto is a constructed or planned language which is actively used by a multifaceted speech community all over the world. For more than a century, the nature of the parts-of-speech (PoS) system of Esperanto has been an item of controversy among its grammarians. Based on a study of the language as it is actually used by its speech community and supported by historical and contemporary documents, the author argues that the language's current PoS system can be described adequately as a mixture of contentive lexemes and verbally specialized lexemes. A system of contentive lexemes, which allows the user to create four different words with predictable meanings out of each single root of any semantic category without having to use derivational tools, does justice to the learnability claim which underlies the structure of this language, which was made to be easy. The increasing number of verbally specialized lexemes is a development which is known to, but unsatisfactorily explained by many esperantologists. The paper develops the hypothesis that the diachrony of Esperanto (constructed in its origin, but freely developing) provides support to the claim that natural languages can be classified in terms of an implicational hierarchy of their PoS systems, in which verbal specialization precedes all other forms of syntactic specialization of lexemes. The analysis is done within the framework of functional discourse grammar.