After an enumeration of the most characteristic features of the Orchidales, the affinities between this order and other monocotyledonous groups are discussed. The Orchidales must be placed in the system immediately after the Commelinales, chiefly on account of their androecial morphology. For reasons explained in the present paper the Orchidales are subdivided into three families, viz. the Apostasiaceae, Cypripediaceae and Orchidaceae. The principal differences between these families are shown in the form of tables. In the Orchidaceae two subfamilies are recognised and amply circumscribed, viz., the Orchidoideae and the Epidendroideae, the latter again being subdivided into the two contribes Neottianthae and Epidendranthae. The androecial morphology is discussed in detail in connection with atavistically developed stamens, with the presence of staminodes (or supernumerary stamens) and with the floral morphology of certain taxa. The auricles are not regarded as staminodes because they persist in flowers with supernumerary (atavistically re-appeared) stamens and also because they lack vascular bundles. The gynostemial wings in some Australian genera (Diuris, Prasophyllum, and other ones) are considered to be of androecial derivation The less conspicuously winged but vascularised margins of the gynostemium of many Epidendroideae likewise represent androecial elements incorporated in the column. The suggestion, made by E. Nelson, that the lip is the phylogenetic derivative (the homologue) of the ‘missing’ stamens A 2, as and As (which implies that the median petal has disappeared), is rejected, primarily because Nelson’s arguments are based on the morphology of the labellum in the highly specialised genus Ophrys. The floral morphology of Neuwiedia and of some species of Herminium, on the contrary, is indicative of the petaloid nature of the lip which, accordingly, represents the median petal. The vascular anatomy of the orchid flower does not provide arguments in favour of the suggestion made by Lindley (later taken up by Darwin) that the ‘missing’ stamens Aa and As are incorporated in the gynostemium. The orchidalean ovary appears to be compounded of six parts, viz. three broad hyposepalous zones alternating with as many hypopetalous elements bearing the (double) laminal placentae on their inner face. This is considered to be a convergence towards the condition prevailing in Rhoeadales (Brassicales) (genus Eschscholzia etc.). In the Orchidoideae the stigma is locally differentiated into viscid discs which are situated distally of the lateral stigmatic lobes, and the median stigmatic lobe is functional in several genera, but in the Epidendroideae the median stigmatic lobe is apically transformed into a mucilaginous substance or a viscid disc, or it is completely replaced by a viscidium. In Stereosandra the viscidial element is a derivative of the anther.