The vegetation and flora of the Sonoran Desert are among the most thoroughly documented of the arid portions of North America. The climate and relative accessibility of the area have attracted many botanists and the landmark studies resulting from their efforts, too numerous to list here (see Shreve and Wiggins 1964, and Kearney and Peebles 1960 for partial bibliographies) are a testament to their achievements. In an area covering over 310,000 square km (Shreve 1951) it is not, however, surprising that some less-known localities should exist, which might harbor plant species not previously known to exist there. For example, H.S. Gentry (1972) described two very distinctive Sonoran Desert species of Agave (A. zebra and A. pelona) from localities in the mountains near the Gulf of California. The plant records reported here were encountered during visits to observe these endemic century plants in a small range of mountains, the Sierra del Viejo.

Taxonomists have published large numbers of scientific articles, monographs and books attempting to classify the creatures which live on earth. Paradoxically, although form (morphology) has been the criterion most widely used by taxonomists to separate the various types of life (creatures) one from the other to produce classification schemes, relatively little attention has been devoted to classifying "life forms" per se. Perhaps this has resulted from a tendency to emphasize phylogenetic reconstruction in preference to the importance of form in relation to function in life. Indeed taxonomists have traditionally studied preserved (dead) specimens from which it can be notoriously difficult to make interpretations relating to functional adaptations. The classification of life forms is only superficially taxonomic. To classify them it is necessary to understand them. To understand them we need to know about their physiological ecology.