Let us make an in-depth study of the modifications and duration of the calyx.
The lowermost axillary whorl is the calyx. An epicalyx sometimes develops below the calyx but it is formed of bracteoles and is not actually a part of the flower.
Sepals forming a calyx are usually green, somewhat resembling abnormal leaves. When the flower is in the bud stage it is protected by the calyx and, to be able to perform this function better, it is sometimes quite thick and hardy although it may show veins and stomata like ordinary leaves.
Like the entire flower the calyx may be regular or irregular. The sepals may be quite free from one another when the calyx is termed polysepalous or dialysepalous as seen in mustard or water-lily flowers. In other flowers the sepals are wholly or partially united with one another. Such a calyx is gamosepalous. There may be different degrees of union.
It may involve only the bases, of the sepals (partite), or the fusion may extend up to the middle (cleft), or the union may extend almost up to the top when he sepal tops are seen as some teeth as in Datura and the condition is described as toothed.
In the extreme case the fusion is complete (entire), the individual sepals being no longer distinguishable. In a united calyx the united portion is called the tube and the free portion the limb.
Sometimes the number of teeth on the limb does not correspond to the number of sepals as the teeth may be further subdivided. Forms of regular gamosepalous calyx may be tubular (Datura), bell-shaped (china-rose), urceolate (Hyoscyamus ), etc., which are the same as explained in connection with corolla. Irregular gamosepalous calyx also may similarly be bilabiate (Leonurus and Leucas ), etc.
Sepals are usually devoid of stalks (i.e., sessile) and with entire margins. Incised sepals may, however, be seen in. rose .
The calyx may sometimes be modified or have some appendages. The sepals may be petaloid, i.e., petal-like as seen in Sterculia roxburghii with a scarlet calyx and also in Saraca asoca and Caesalpinia pulcherrima of Caesalpinieae, etc.
In Mussaenda, while four sepals are normal the fifth one is large and petaloid although showing vein marks. In the family Compositae and the allied families Dipsaceae and Valerianaceae, the calyx is modified into two or more hairy, scaly or feathery structures forming the pappus .
In Trapa bispinosa (water chestnut ) the calyx is spinous in the fruit. In Impatiens balsamina ,Tropoeolum majus, Delphinium (larkspur), etc., the sepal is prolonged downwards into a tubular process called spur and the calyx is said to be spurred.
In Aconitum (monkshood of Ranunculaceae ) one sepal is enlarged forming a hood over the flower.
2. Duration of the Calyx:
The calyx of some flowers drops off as soon as the flower opens as in poppy , Argemone mexicana, etc. This type of calyx is termed caducous or fugacious. In many flowers the calyx is, deciduous, i.e., it falls off along with the petals just after fertilization as in mustard, etc. In brinjal, Datura, Ocimum, Trapa , etc., the calyx is persistent and remains attached to the ripe fruit.
In inferior fruits like guava, the calyx often persists on the fruit. If a persistent calyx assumes a shrivelled dried-up appearance as in guava, it is termed marcescent. In a number of fruits like those of Dillenia indica of Dilleniaceae, Physalis of Solanaceae , Shorea of Dipterocarpaceae , etc., the calyx is not only persistent but also grows in size along with the fruit.
Then it is called accrescent. In Dillenia the sepals are fleshy containing much arid and mucilage. These are edible.