In their monumental work The Cactaceae , BRITTON and ROSE (I9) record 1235 species belonging to the three tribes which constitute the family of the Cacti. The actual number of the species must be con- siderably higher. Cacti occur frequently in tlie more arid and less accessible regions of the American Continent, nearly always within very narrow and definite borderlines. The habitat of a species is in many instances a single valley located in a remote, uninhabited region of the Cordillera. Thus the collection of flowering specimens fit for botanical identification is some- times extremely difficult. On the other hand, cacti are apt to develop individual variations in their characteristic morphological features, rendering the definition of a species difficult and often illusory. Specimens taken from their normal habitat to botanical gardens or arboreta often die, degenerate or stop flowering. Taking into account all these difficulties, it is not surprising to find considerable differences of opinion among botanists on the taxonomy of the cactaceae. A considerable number of species have not been well defined and in many cases different names have been given to the same species. The index of BRITTON and ROSE records not less than 7000 binomials.