The union of the calicinal foliola sometimes forms a conical calyx, as in the Silena conica, and sometimes a cup-shaped calyx, as in the orange; sometimes, moreover, an urceolate calyx, as in the henbane (Hyosciamus niger). These forms may vary singularly. The calyx of the black alder (Rhamnus frangula) is shaped like a top; that of the haricot (Phaseolus vulgaris), like a bell; that of the tobacco-plant, or the Mollucella spinosa, is infundibuliform (or funnel-shaped).
Fig. 45.—The Henbane.
The observer is sometimes embarrassed in deciding to which whorl he should refer the foliola he is examining. Thus, the tiny foliola which, in the strawberry and the potentilla, alternate with others and larger ones, are stipules rather than sepals. Ought the sepals of the calyx in the Malvaceæ to be assimilated in like manner to the stipules? It is difficult to reply to this question satisfactorily. Take, for example, the Hibiscus Syriacus, an ornamental shrub, better known by the name of the garden-hemp. The inner calyx, or calyx properly so called, of this Malvacea has five sepals, while the outer calyx, or calicule, has twelve. Now, a leaf cannot have more than two stipules, one on each side. For an outer calyx, then, the proper number of foliola is ten, not twelve. To look upon the second calyx as a "supernumerary development," would be to hazard a supposition contrary to the unity of plan of the floral organs.
The calyx, like the corolla, is not an absolutely indispensable organ. Sometimes, therefore, it is caducous—that is, falls off before the flower expands,—as in poppies; sometimes, persistent, or remains after flowering,—as in roses and the majority of plants. In some cases it is persistent only until after the act of fecundation, but this act accomplished, it falls with the corolla in most of the Cruciferæ and Ranunculaceæ. This is a deciduous calyx.
The "caducity" and "persistency" of floral envelopes furnish some valuable characteristics for the distinction of species. Thus, two closely-allied Cruciferæ, the Alyssum calicinum, so common in spring upon stony soils, and the Alyssum montanum, can only be distinguished from one another by the fact that the calyx of the former is persistent, of the latter caducous. It is true that the flowers of the Alyssum calicinum are of a yellow which easily passes into white, while those of the Alyssum montanum are of a beautiful permanent yellow. But this latter distinction is not so good as the former.
The persistent calyx sometimes assumes a considerable increase of very common appearance. For example, take the Physalis alkekengi, a member of the Nightshade family or Solanaceæ. The red bladder-like accrescence surrounding the scarlet fruit is the calyx, which, after flowering, has grown much larger than it was before. And these bright flowers which resemble large strawberries, and abound on the borders of meadow-paths, if you look at them closely, you find to be the accrescent calices of the Trifolium fragiferum. The very word fragiferum reminds us of the strawberry.
The calyx may change in consistency and texture in proportion as the ovary, to which it adheres, changes into fruit. The fleshy pulpy substance of the apple, and, in general, of the fruits of the Pomaceæ, is simply an excessive development of the calicinal tube united all around the ovary, and recognisable in the pips, imprisoned, towards the centre, in horny lobes. In other plants, as the flower develops into fruit, the calyx becomes woody: such is the case with the Water Chestnut (Trappa natans).
Finally, the calyx may even contribute to the dissemination of the seed. We may cite, as an example, a Brazilian species of Urticaceæ, which Saint-Hilaire named the Elasticaria. The fleshy and cylindrical parts of the calyx are curved inwards, and thus defend, as one might do with one's bended fingers, the young fruit until it is completely developed; as soon as the fruit is ripe, they spring up erect, and launch it to a distance.