Complex sentence: They are known for their effusive flowered heads (usually purple, rose, yellow, or white) and prickly stems and leaves, with a characteristic focal point at the base of the flower which is particularly spiny.
Split into simpler sentences: They are known for their effusive flowered heads (usually purple, rose, yellow, or white). They have erect, prickly stems and leaves, with a characteristic focal point at the base of the "flower" which is particularly spiny.