Standard petal of a papilionaceous corolla is known as

The correct option is vexillum.

To understand why this is correct, let's break down the structure of a papilionaceous corolla (the collection of petals in a pea-like flower):

1. Papilionaceous Corolla: This is a characteristic butterfly-shaped arrangement of petals found in plants belonging to the family Fabaceae (or Leguminosae). It consists of five petals.
2. The Standard Petal (Vexillum): The largest, outermost, and topmost petal is known as the standard or vexillum. It acts as a banner or flag to attract pollinators like bees.
3. The Wing Petals (Alae): Below the standard petal, there are two lateral petals called the wings or alae.
4. The Keel Petals (Carina): The two smallest, innermost petals situated at the bottom are fused together to form a boat-shaped structure called the keel or carina, which encloses the stamens and pistil.

Now, let us examine the other terms mentioned in the options to see why they do not represent the standard petal:
• Carina: This is the term for the two fused keel petals at the bottom, not the standard petal.
• Corona: This refers to a cup-like or horn-like outgrowth of the corolla or stamens found in other plant families (such as in daffodils or milkweeds), rather than a specific petal in a papilionaceous flower.
• Pappus: This is a modified calyx (sepals) represented by bristles, hairs, or scales in the family Asteraceae, which helps in wind seed dispersal.

Thus, the standard petal of a papilionaceous corolla is specifically called the vexillum.