The genera _Caltha_, _Anemone_, _Thalictrum_ and _Clematis_ have a _single perianth_, which is most frequently petaloid; it is thus apparent that the sepals are petaloid, and the leaves, which in other genera have developed as petals, are in these instances stamens. The calyx is similarly petaloid in the genera _Helleborus_, _Eranthis_, _Nigella_, _Delphinium_ and _Aconitum_; but the petals are present in these instances in unusual (horn-like) forms, and almost entirely given up to the function of nectaries, a function they already possess in _Ranunculus_. According to a more recent theory the “honey-leaves” are transformed stamens, which have lost the function of reproduction; the perianth is then single, and most frequently petaloid. [Those leaves in the flowers of many Ranunculaceæ which bear nectaries are termed by Prantl honey-leaves, and comprise those leaf-structures of the flower whose essential function lies in the production of nectar, and which, independent of the differentiation of the perianth into calyx and corolla, are derived from the stamens by the loss of their reproductive functions. Clear transitional forms are found between the two series of the perianth (_e.g._ between the sepaloid and petaloid perianth-leaves of _Anemone japonica_, _A. decapetala_, _Trollius_-species) while transitional forms are never found between perianth-and honey-leaves (with the exception of _Aquilegia vulgaris_, var. _stellata_). In _Anemone_ and _Clematis_ the honey-leaves pass gradually into the stamens, and agree with the stamens in the other Ranunculaceæ in their arrangement, development, and scant system of veins (except _Nigella_). In _Delphinium_, sect. _Consolida_, the two honey-leaves placed in front of the unpaired perianth-leaf are united into one, as shown by the veins (twice three veins arranged symmetrically). The honey-leaves of _Aquilegia_, _Callianthemum_, and the majority of the _Ranunculus_-species serve by reason of their large circumference, as organs of attraction, and on this account are considered as petals by other authors.--The same position in the flower which the honey-leaves assume is found occupied by staminodes, without nectar, in some _Coptis_-species, in _Anemonopsis_, _Actæa_ sect. _Euactæa_, (_e.g. A. racemosa_), _Clematis_ sect. _Atragene_; in the last-named they closely surround the stamens, in _Actæa_ they are petaloid.--A perianth, sharply differentiated into calyx and corolla, and destitute of honey-leaves, is found in _Anemone_, sect. _Knowltonia_ (Cape),