The third and last treatise is the _Dialogus_: a dialogue between himself, as _Botanographus Regius_, King's Botanist, and a Fellow of the Royal Society, on the theme of classification. Here again Morison asserts the superiority of his own method: "_Methodum me observasse fateor: estque omnium quae unquam adhuc fuerunt exhibitae, praestantissima et certissima quippe a natura data_." But he still fails to give any definite account of it: all that he says amounts merely to this, that the "_nota generica_" is not to be sought in the properties of a plant, nor in the shape of its leaves, as had been suggested by earlier writers, but in the fructification, that is, in the flower and fruit (_essentiam plantarum desumendam ... a florum forma at seminum conformatione_). The mention of a system of classification based on the form of the leaf evoked from _Botanographus_ a pointed allusion to a book recently published by a Fellow of the Royal Society in which such a classification had been used, with the following severe comment: "_Ego tantum confusum Chaos: illic, de plantis legi, nec quicquam didici, ut monstrabo tibi et lapsus et confusionem, alias_." The book so criticised was the encyclopaedic work edited by Dr John Wilkins, Bishop of Chester, and published by the Royal Society in 1668, entitled, "_An Essay towards a Real Character and a Philosophical Language_," to which John Ray had contributed the botanical article '_Tables of Plants_.' This criticism was the beginning of the unfriendly relations between Morison and Ray, of which some further account will be given subsequently. Another point of interest in the _Dialogus_ is the definite assertion (p. 488) that Ferns are 'perfect' plants, having flower and seed (_quia habent flores, qui fugiunt quasi obtutum, et semina quasi pulvisculum in dorso alarum_), an assertion which was repeated with even greater emphasis in Morison's preface to his edition of Boccone's _Icones at Descriptiones Rariorum Plantarum_ etc. (Oxon. 1674), in opposition to the views of earlier writers, Cesalpino in particular. Cesalpino had, it is true, said of the group in which he had placed the Ferns and other Cryptogams, "_quod nullum semen molitur_" (_De Plantis_, p. 591): but he had added, in the same paragraph,--"_ferunt enim in folio quid, quod vicem seminis gerit, ut Filix et quae illi affinia sunt_." It is a question if Morison was much nearer the truth than Cesalpino.