Brassica oleracea Italica group.
Brassica oleracea var. italica – Common broccoli.
Plants, up to 60 or 90 cm tall have heads similar in structure to cauliflowers but instead of aborted flowers broccoli have mature flowers in the clusters.
They are harvested just as the flowers are starting to open so the commonly seen heads are green.
There are other cultivars with white, yellow or violet heads.
There are other forms of broccoli.