Calyx. Although in some flowers the transition from ordinary foliage leaves to the floral envelopes immediately surrounding the stamens and carpels is a gentle and almost insensible one, we can, in the vast majority of cases, recognise the floral envelopes as clearly defined. The outer circle or whorl of these is what we term the calyx, the inner being the corolla; and the separate leaves of the calyx are termed sepals, corresponding to the petals. In most monocotyledons the outer and inner whorl of floral envelopes are essentially similar, and both are usually equally coloured or white; in these cases it is customary to unite them under the common term of perianth. In others, as in most dicotyledons, the calyx is green and leafy, and is sharply distinguished from the corolla, to which the floral colour is thus restricted; in other cases, however, especially when the floral envelopes are in a continuous spiral, as in the water-lily, the transition from leafy sepal to distinct petal is a quite gradual one. The leafy habit of the calyx varies within the widest limits; thus, we have on one hand the vegetative extreme, where the sepals are not only leafy in the flower, and perhaps even possess stipules (strawberry), but persist after the corolla has withered, and even grow up around the fruit, as in the winter-cherry (Physalis). Next, we have the most frequent case of ordinary green sepals, which wither or fall off when flowering has been accomplished, or they may drop off at a still earlier stage, when the flower opens, as in the poppy; or again, they may become petaloid, especially when the petals are reduced to nectaries, as in the Christmas rose or monkshood, or have disappeared altogether, as in the marsh-marigold. In the monochlamydeous orders of old classifications—e.g. Polygonaceæ, Chenopodiaceæ—the calyx also alone persists, but is small and inconspicuous; while in the so-called achlamydeous types this also disappears—e.g. willow. In such cases the protective functions of a calyx are performed by bracts, and these again often become modified and reduced in their turn. Thus in grasses, the characteristic protective glumes are not sepals, as was at first supposed by botanists, but bracts, the perianth being represented only by a couple of tiny scales. A similar modification holds good in the allied sedges (Cyperaceæ).
The calyx is said to be inferior when, as in typical unmodified flowers, it occupies its normal place lower on the axis than the other parts, and particularly the ovary; in many cases, however, it is superior—i.e. borne upon the summit of the fruit, as in the apple or rose. The explanation of this used to be that an ‘adherence’ had taken place between the inner surface of the calyx and the outer surface of the ovary, and the term adherent was therefore frequently applied to it. Not only, however, is there no evidence in favour of this explanation, but we know, alike from the study of development, from comparative anatomy, and from physiology, that the inferior ovary comes into its place by the checking of the apical growth of the floral axis, which thus becomes cupped instead of pointed. This hollowing of the summit of the axis naturally begins at the point where the leafy sepals and expanded petals mark the limit where vegetation ceases, and the essential reproductive organs take the place of less modified foliar ones. The apparent sinking of the ‘inferior’ ovary is, in short, the consequence of the onward growth of the ‘superior’ calyx, the portion of the axis which bears it. See FLOWER.