Stanford Arizona garden - Opuntia

Here is an Opuntia composition, from my last visit to Stanford cactus garden. Late light of the fall day  gives the pad this both gentle and crisp touch, making photographer's job easy. This is a tall large-padded Opuntia, I have to look up it's name some day.

Notocactus ottonis vencluianus

Notocactus (Parodia) ottonis 'vencluianus' is considered a flower color variation of common yellow-flowering species. The plant stem does look a lot like that of N.ottonis, but I've not heard of successful crossing of these plants. Than may indicate that this is actually a different species all together, another example of converging evolution in Cactaceae.
The flower color is somewhat too bright and almost hurting to look. It is somewhat unclear to me why this form is not seen more often in collections, as growing it not any different than growing any other Notocactus, and seeds are readily available from many good sources.

Pediocactus simpsonii

Pediocactus simpsoni is a widely spread species, occurring through USA areas from Rocky Mountains and west. It is adopted for the environment where one would not expect to see cactus - areas with permanent winter snow cover, where pine trees not yuccas grow.
The plant is pretty easy in culture, once it's requirements understood. Pedoicactus simpsonii gets most of it's year's water supply from snow melt. It blooms on southern slopes at the time when nearby north facing sides of the same hill are still under snow, and air is cool.

Grusonia clavata

Genus Grusonia was created  for small North American Opuntoides that distinguish from Cylindropuntias by lack of papery spin cover - 'sheath'. They are low spreading plants, often forming mats, and easily noted for downward pointing spines. Grusonia clavata is more often than other species seen in collections, for it's compact growth and dagger-like spines.
Grusonia buds are also easily recognized for they shape with distinct long 'tube'. The plant in picture had only two mature segments a that time, and was ready to flower at the size.

Echinocereus sciurus floresii

The closeup of A typical Echinocereus flower - green stigma, bright petals. Flowers ate large and wide open, making pollination easy, either natural or artificial - with a brash.All Echinocerei are self sterile and require second plant and cross-pollination o develop seeds. 

Mammillaria vetula 'gracilis"

M. gracilis is very common plant in Europe, often the first cactus in collection. It is offsetting freely, and readily roots. When grown under plenty of light this plant forms very attractive dense white clumps, with stubby black central spines. Fruits are rarely seen, and are green in color.

Mammillaria pertophila

Mammillaria pertophila is Baja California species, from the Southern tip of peninsula. It is a small in nature, clumping in culture plant noted for its dense wool and greenish flowers. Larger plants look nice with lots of white wool in axiles. 

This species is pretty easy in culture, but not that commonly seen, usually only in specialized collections.