In some flowers calyx and corolla are undifferentiated and cannot be distinguished. They are termed perianth, e.g. Lily.

Perianth:
Perianth refers to the outermost part of a flower, which is composed of the calyx and corolla. It can be described as the non-reproductive, protective covering of a flower. The perianth is usually composed of separate or fused units called tepals, which are similar in shape and color.

Constituents of Perianth:
The constituents or members of the perianth are known as tepals. Tepals are the individual units that make up the perianth and are often indistinguishable from each other in terms of shape and color. In some flowers, the calyx and corolla are differentiated, with the calyx being the outermost whorl and the corolla being the inner whorl. In such cases, the calyx is composed of sepals, and the corolla is composed of petals.

Example:
An example of a flower with a perianth composed of tepals is the tulip (Tulipa spp.). In tulips, the perianth consists of six tepals, which are usually brightly colored and petal-like in appearance. These tepals protect the reproductive organs of the flower and play a role in attracting pollinators.

Importance of Perianth:
The perianth serves several important functions in a flower:

1. Protection: The perianth protects the reproductive organs of the flower, including the stamens and pistil, from physical damage, desiccation, and pathogens.

2. Attraction: The perianth, particularly the corolla, often plays a role in attracting pollinators. Brightly colored and fragrant perianth structures can attract insects, birds, and other animals, which aid in the transfer of pollen.

3. Adaptation: The perianth can also be involved in adaptation to specific ecological conditions. For example, certain plants have perianth structures that are adapted to withstand extreme temperatures, drought, or other environmental stresses.

4. Variation: The perianth is highly variable in terms of shape, size, color, and texture, contributing to the diversity of flowers. This variation is important for species recognition, hybridization, and evolution.

In summary, the perianth is the outermost part of a flower composed of tepals. Tepals are the individual units that make up the perianth and can be indistinguishable from each other. The perianth serves important functions in protecting and attracting pollinators, adapting to ecological conditions, and contributing to flower diversity.