Which classification of bonsai is characterized by a vertical growth style?

The classification of bonsai known for its vertical growth style is the formal upright. This style, referred to as "chokkan" in Japanese, typically presents a straight, vertical trunk that extends directly upward. The branches are arranged in a way that enhances this verticality, often being shorter as they ascend, which creates a sense of balance and harmony.

In formal upright bonsai, the overall appearance is designed to convey strength and stability, resembling an idealized natural tree. The tree is usually well-proportioned, and the foliage at the apex can create a distinct focal point. This classification is often chosen for its classic aesthetics and is one of the most recognizable forms of bonsai.

The other classifications, while they each have their own distinct styles and characteristics, do not specifically emphasize vertical growth in the same manner as the formal upright.