There are at least 1,650 species of cactus that occur in a great variety of forms: small round globes, vines, flat-stemmed ground-sprawling lobes, and enormous tree-like structures. Cactuses have delicate, but usually large, solitary flowers in shades of yellow, white, red and purple. Not all cactuses, however, bloom in the spring. The Christmas cactus Schlumbergera bridesii opens its blossoms after a period of cool temperatures or long nights, generally in the late fall or early winter. Some cactus blossoms are light sensitive and can only be seen at night or for a matter of hours. Flowers can be pollinated by wind, birds, insects, or bats.