How do Brussels sprouts form on a Brassica oleracea plant?

On a Brassica oleracea plant, Brussels sprouts form through the development of the plant's axillary buds along the main stem. As the plant matures, these buds grow into small, compact heads that resemble miniature cabbages, typically measuring 25 to 40 mm (1 to 1.6 inches) in diameter.1

While the plant initially resembles common cabbage during its seedling and early development stages, the main stem of a Brussels sprout plant eventually reaches a height of 60 to 90 cm (2 to 3 feet), allowing these sprouts to develop along its length.1