Why Do Flowers Have Sepals?
The sepals of a flower protect it while it is in bud. The flower is really a kind of shoot, in which the leaves have been altered so that they can take on the task of producing seeds. In a simple flower these leaves are arranged in circles, called whorls. The outermost are five green, leaf-shaped sepals.
Inside these are five petals, usually heart-shaped, each with a small flap at its base where nectar is produced to attract bees and other insects. Both the sepals and the petals are attached at their bases to the “receptacle”, the swollen end of the flower-stalk, which look like a cone in the middle of the flower.
Above the sepals and petals are the parts of the flower used in reproduction. These are the stamens, which contain the yellow pollen, and the carpels, which contain the ovules. Most flowers are built on this plan but there are wide variations in size, shape and color, and in the numbers of the different parts of the flowers.
Morphologically, both sepals and petals are modified leaves. The calyx (the sepals) and the corolla (the petals) are the outer sterile whorls of the flower, which together form what is known as the perianth.
The term tepal is usually applied when the parts of the perianth are difficult to distinguish, e.g. the petals and sepals share the same color, or the petals are absent and the sepals are colorful. When the undifferentiated tepals resemble petals, they are referred to as “petaloid”, as in petaloid monocots, orders of monocots with brightly coloured tepals.
Since they include Liliales, an alternative name is lilioid monocots. Examples of plants in which the term tepal is appropriate include genera such as Aloe and Tulipa. In contrast, genera such as Rosa and Phaseolus have well-distinguished sepals and petals.
The number of sepals in a flower is its merosity. Flower merosity is indicative of a plant’s classification. The merosity of a eudicot flower is typically four or five. The merosity of a monocot or palaeodicot flower is three, or a multiple of three.
The development and form of the sepals vary considerably among flowering plants. They may be free (polysepalous) or fused together (gamosepalous). Often, the sepals are much reduced, appearing somewhat awn-like, or as scales, teeth, or ridges. Most often such structures protrude until the fruit is mature and falls off.
Examples of flowers with much reduced perianths are found among the grasses. In some flowers, the sepals are fused towards the base, forming a calyx tube (as in the Lythraceae family, and Fabaceae). In other flowers (e.g., Rosaceae, Myrtaceae) a hypanthium includes the bases of sepals, petals, and the attachment points of the stamens.