Which type of flower is characterized by having single stems with large rounded heads, commonly used in arrangements?

The type of flower characterized by having single stems with large rounded heads is classified as mass flowers. These flowers are known for their ability to create focal points in floral arrangements due to their bold and abundant blooms. Mass flowers are used to add volume, form, and a sense of fullness to an arrangement, making them essential in creating visually impactful designs. This is especially true in wedding bouquets, centerpieces, and other arrangements where a significant presence is desired.

Examples of mass flowers include roses, peonies, and hydrangeas, all of which have prominent heads that can dominate a floral composition. They differ from line flowers, which have elongated shapes that help create structure in an arrangement, and filler flowers, which are used to fill in gaps and add texture. Base flowers, while not a standardized term in floral design, usually refer to those forming the foundation of a floral piece, but are not recognized as a distinct category comparable to mass, line, or filler. Thus, the classification of mass flowers is best suited for the description given in the question.