Desert Plants, Volume 5, Number 4 (Winter 1984) Item Type Article Publisher University of Arizona (Tucson, AZ) Journal Desert Plants Rights Copyright © Arizona Board of Regents. The University of Arizona. Download date 23/09/2021 10:47:41 Link to Item http://hdl.handle.net/10150/552238 Volume 5, Number 4 Desert Published by The University of Arizona for the Plants Boyce Thompson Southwestern Arboretum. Editorial -Life Forms of Desert Plants 130 A Classification of Life Forms of the Sonoran Desert, With Emphasis on the Seed Plants and Their Survival Strategies 131 F. S. Crosswhite and C. D. Crosswhite The Acanthaceae of the South- western United States 162 T. F. Daniel New Plant Records From the Sonoran Desert 180 G. Yatskievych and P. C. Fischer Publication of Dr. Howard Scott Gentry's Book Agaves of Continental North America by the University of Arizona Press 191 Crassulacean Acid Metabolism 192 Tallest known Boojum Tree (Idria columnaris), over 81 feet high, with a massive Cardón Pelón (Pachycereus pringlei) at Montevideo Canyon, north of San Borja, Baja California del Norte. Photo by Mark Dimmitt. See article on life forms, pg. 131. 130 Desert Plants 5;41 Winter 1984 Volume 5, Number 4, 1984 Desert Plants Published by The University of Arizona for the Boyce Thompson Southwestern Arboretum A quarterly journal devoted to broadening knowledge of P.O. Box AB, Superior, Arizona 85273 plants indigenous or adaptable to arid and sub -arid regions, to studying the growth thereof and to encouraging an The Boyce Thompson Southwestern Arboretum at Superior, appreciation of these as valued components of the landscape. Arizona, is cooperatively managed by The Arizona State Parks Board, The Boyce Thompson Southwestern Arboretum, inc., and The University of Arizona. Frank S. Crosswhite, editor Editorial Life Forms of Desert Plants. With this issue we present germplasm, we have the distinct opportunity of breeding new an illustrated classification of the life forms of Sonoran Desert life form characteristics into already established crop plants seed plants. These "life forms" are actually much more than the originating outside of the desert! simple hodge -podge of shapes and structures that they might By eventually understanding the successful "ways of living" at first appear. In reality they represent important "ways of of plants in the desert we will have a much firmer basis for living" which we might term strategies for survival. In a world genetic engineering than would have otherwise been possible. too often filled with pessimism, our minds too frequently are A platycaulescent shrubby stem succulent can produce large flooded with talk of economic uncertainties, sinking water quantities of sugar and biomass under agriculturally unfavor- tables, escalating utility rates, possible oil embargos, and fear able conditions. Although we may not particularly like the of "The Day After." But a consideration of desert life forms biomass that any one species in this life form produces, we speaks to us only of optimism. We dare our subscribers to read need to look beyond species to the life form itself, -there the following article and feel anything other than a sheer sense would seem to be no reason why we should not manipulate the of exhilaration for the triumph of the various life forms over life form to produce something which we do want! the harsh and rigorous hazards that they have faced. The few life forms which we fully exploit today happen to Not only have plants been able to adapt to the Sonoran be ones which our agricultural ancestors domesticated Desert, but they have found at least 29 major ways to do so! We ;mostly in temperate regions) in the dim and distant past. are only now beginning to scratch the surface in understand- Although the plants of the Sonoran Desert have existed for ing how these life forms function. We have thus far signifi- millenia, it is our present generation and those of the future cantly exploited only two or three such life forms for food, which will bring an appropriate blend of technology and fiber, energy and other useful goods. True, many of the other understanding to bear on the fascinating possibilities of life forms have been utilized in some way by Indians and early obtaining good from these fantastic life forms! This origi- settlers. But the fantastic germplasm of most life forms has not nally was, and continues to be, the most basic goal of the been manipulated in the slightest by plant scientists! And Boyce Thompson Southwestern Arboretum. even beyond what nature has provided in Sonoran Desert Cross white and Crosswhite Desert Life Forms 131 Taxonomists have published large numbers of scientific A Classification of 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 Life Forms of the taxonomists to separate the various types of life (creatures) one from the other to produce classification schemes, rela- Sonoran Desert, With tively 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 Emphasisonthe Seed importance of form in relation to function in life. Indeed taxonomists have traditionally studied preserved (dead) speci- Plants and Their mens from which it can be notoriously difficult to make interpretations relating to functional adaptations. The classi- Survival Strategies fication of life forms is only superficially taxonomic. To classify them it is necessary to understand them. To under- stand them we need to know about their physiological ecology. In this article, "life form" is not used as a synonym for "creature" or "organism," but rather in the sense of "form of life," Frank S. Crosswhite or even "form of living," with clear functional, physiological and Carol D. Crosswhite and ecological connotations. The distinctions between a life form classification and a phylogenetic one may sometimes be Boyce Thompson Southwestern Arboretum blurred. Although there is no theoretic reason that they should (or should not) necessarily coincide, sometimes they do. Monophylesis results when a single life form diverges into two over time. Polyphylesis occurs when separate life forms converge into one. Since phylogenetic classifications allow only for monophylesis (unless actual hybridization has oc- curred), they are based on divergence rather than convergence. Therefore a life form classification will diverge from a phy- logenetic classification in the degree to which the life forms themselves have undergone convergence. Likewise, the two kinds of classifications will converge in the degree to which the life forms themselves have undergone divergence. The distinctions between a life form classification and a phylogenetic one are perhaps easier to see in the plant king- dom than in the animal kingdom. For instance, numerous plant families have independently given rise to species with the typical succulent life form based on crassulacean acid metabolism (see back cover of this Desert Plants). Like- wise, distantly related plant families have independently given rise to species with the deciduous tree life form based on leaf abscission and massive secondary xylem formation. Sometimes people are startled to learn that a tiny weed is hereditarily more closely related to some huge tree than that tree is related to another huge tree which superficially looks the same! In such a case the two trees may have an identical life form while having quite different genes and chromosomes. The hydrophytic algal life form also appears to be highly polyphyletic. Parasites have arisen in many independent plant lines. We could go on for many pages with similar examples. Among animals, life forms are perhaps somewhat more monophyletic. For example, we know of no fish which have developed feathers, no parasitic elephants, no swimming flies, no birds which burrow through soil like earthworms, no snakes with wings, no flying turtles, etc. True, penguins, whales and bats have departed from the respective norms of their relatives, converging to some extent with other life forms, but they seem to be exceptions to the general state in the animal kingdom. Therefore, a classification of animal life forms perhaps coincides more closely with their true phylo- geny than does a similar classification of plant life forms. Representative hot -season therophyte: Summer Representative bulb geophyte: Mariposa Lily Poppy (Kallstroemia grandiflora) and Trailing Four O'clock (Calochortus kennedyi) at King Canyon near Tucson, (Allionia incarnata) in the San Simon Valley, southern Arizona. Photo courtesy Arizona -Sonora Desert Museum. Arizona. Photo by Mark Dimmitt. Al Morgan, photographer. Representative cold- season therophyte: Cream - Representative facultatively chamaephytic Cups (Platystemon calffornicus) at Arboretum Pass therophyte: Desert Beardtongue (Penstemon parryi), between Arnett Canyon and Queen Creek Canyon, Boyce showing increased vigor over surrounding therophytes Thompson Southwestern Arboretum. Photo by Carol D. (Cryptantha, Lupinus), Pinal Pioneer Parkway, Arizona. Crosswhite. Photo by Carol D. Crosswhite. Crosswhite and Crosswhite Desert Life Forms 133 Scientists have been a long time in arriving at an under- ignore physiology and ecology (perhaps rightly so if the key is standing of the underlying factors relating to a life form classi- merely for identification purposes) that we tend