Amaryllis is the type genus for the family Amaryllidaceae. Although it is the common name used for hybrid hippeastrums which are found on the Hippeastrum Hybrids page, there are actually only two species in this genus and one of them was only recently discovered. Both are found in the winter rain area of South Africa.
Many Amaryllis selections have been made, and the species has also been hybridized very heavily with related genera for more than a century. To learn more about the history of some of these hybrids, read this conversation between David Sneddon and Jim Lykos, two Australian members of the PBS list. An example of these is xAmarcrinum, a hybrid between Amaryllis and Crinum. See also xAmarygia.
Amaryllis belladonna, known to many as "Naked Ladies" because the flowers are pink to white and bloom before the leaves develop, is found in loamy soils in seasonally moist sites in the Cape Floral Province. It has a great variation in time of bloom, starting in summer and extending into fall. It has naturalized in many mediterranean climates throughout the world and is especially popular in California.
A second species, named Amaryllis paradisicola, was described in 1998. It is not in general cultivation. A short description and link to a photo can be found here.
Many people confuse Lycoris squamigera and Amaryllis belladonna because both bloom in the fall on bare stalks with no leaves, and both can have large pink flowers. You can see the differences on the Amaryllis belladonna versus Lycoris squamigera page.
There have been many discussions on the PBS list about how to get Amaryllis belladonna to bloom. Members living in Mediterranean climates are much more likely to get it to bloom. People have said the bulbs need to stick out of the ground and they have also said they need to be covered. Some summer water may help, but you also hear a dry dormancy is helpful. Plants in the ground bloom better than plants in pots and plants growing in the sun are much more likely to bloom than ones in the shade. One factor that influences blooming in the wild is fire. After a summer fire, there is usually good bloom in the fall in the wild. In September 2009 a restaurant burned to the ground in northern California denuding the slope below which was covered with low growing vegetation. Several weeks later spikes were appearing. It's hard to know when or if these bulbs were planted since the slope is steep and it would seem to be impossible to plant them. The photos from Mary Sue Ittner don't really capture how steep the slope is since I had to take the pictures with my zoom lens from the bottom. The third picture was taken the following month when more flowers were open. In 2012 the slope is now covered with non native weedy vegetation and the plants are no longer flowering.
An article in Veld & Flora magazine described the plants' strong response to fire in the wild, and speculated that they bloom more frequently in cultivation because weeds are removed from around them. The article has been reproduced here.
The first photo by Bob Rutemoeller shows some in bloom September 2004 on the Mendocino coast in northern California. Photos 2-4 are by Nhu Nguyen. Photo #2 is from a yard in Berkeley, CA and #3 from the UC Botanical Garden showing these plants blooming dramatically en masse. The plants from the UC Botanical Garden bloomed in an early year in mid July, 2008. Photo #4 is a closeup. The last photo from Mary Sue Ittner is a close-up of one flower.
Photos from Bob Rutemoeller below show blooms along the Mendocino Coast August 2014 where they are growing in full sun with no summer water.
Photographs by David Pilling with the first one of (less than flowering size) bulbs four years after the seed was sown (the coin is about an inch in diameter) and the second showing a flower emerging. Photos 3 and 4 are of commercially supplied bulbs of the white variety on a 10 mm grid.
David Pilling describes his experience of buying three bulbs on ebay in the following paragraphs. Apparently a large clump was being dug up and I thought they might flower well if they had survived in someone's garden for some time; every piece of root had been removed from the bulbs and they took time to settle down. It was reasonable to put them all in an 8" pot. In 2012, a year with a spring and summer dark, cold and wet by even the demanding standards of the North of England, one bulb flowered in the middle of October (the bulbs had gone dormant in August). It came as a surprise to me that the flowers are fragrant.
In the first photo the top of the tape is 18" above the surface of the soil. The next photo shows how the flower colour changes (old flower on the left). Photo three is of unopened anthers; photo four shows them in various stages of opening. The last two photos show the tip of the style and the style with with pollen applied.
In 2013, two bulbs produced flowers. The photos below show detail from the tips of the flower petals. A question to the PBS list as to what these structures did and were called, brought forth a comprehensive reply from Dylan Hannon. They are called "cohering keels" and keep the flower closed until it has developed fully. The classic paper on the subject is "Cohering Keels in Amaryllids and Related Plants" by H. Harold Hume Proceedings of the Florida Academy of Sciences (1: 48-57, 1936). More photos can be seen for Nerine bowdenii.