Opuntiads of the USA

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Dave Ferguson writes:

Opuntia davisii is a "species" that is known as apparently only two clones that reproduce vegetatively.  The more widespread "yellow-spined" clone is found on the Great Plains from southwest Oklahoma, the Texas Panhandle, and northeast New Mexico, southward apparently to near the Balcones Escarpment in western Texas and across central and southwest New Mexico into northwest Chihuahua (mostly in isolated colonies).  It is found in grasslands on sandy (often straight sand) soils.  It is the one shown in all of these photos except the one from Gary Nored, which appears to be of the Trans-Pecos clone. 

The red-brown clone is found apparently only in grasslands south of the Davis Mountains in Texas (Jeff Davis, Brewster, and Presidio Counties), and perhaps into Chihuahua.  It also favors sandy soils in grassland; it looks and behaves pretty much the same but has brownish spine sheaths, and (as I recall) the flowers also have a bit more brownish or purplish pigment to make them look a bit "dirtier".  Again, I've personally seen no seeds on that clone either.  Apparently the Great Plains clone is tetraploid (2n = 44), while the Trans-Pecos clone is diploid (2n = 22).  I've never tried to get the two together to see if they will make seeds if cross-pollinated.  A good place to see the Trans-Pecos clone (if my memory is correct) is the Fort Davis cemetery.

It might be as simple as they need a mate of the same species.