Present Problems in the Anatomy, Morphology, and Biology of the Cactaceæ (Concluded)

Abstract

In this paper I purpose to discuss briefly the subject outlined by the above title, pointing out in particular those questions which can be settled only by study in the field, as well as those which require specially-collected field-material for their solution in the laboratory. The subject can be the more clearly understood and its importance the better judged if I give first a brief description of the anatomical, morphological, and biological characteristics of the family, and then add a short account of progress to our present state of knowledge. The Cactacee form a sharply-defined, although phylogenet-ically very new, practically entirely American order, including some i,ooo usually badly-defined species grouped in some twenty worse-defined genera. Taken as a whole they exhibit a more extreme deviation from the normal in habits, and therefore in structure, than is to be found in any other large family of flowering plants; they offer in consequence many inviting problems, and as well an unusually favorable opportunity to test some of the great principles which are concerned with the nature of adaptation and the dynamics of development. For the most part the Cactaceae are dwellers in the desert and therefore economizers of water. To store water and to protect it from evaporating under the too-great power of the sun, requires a condensed form and this characteristic dominates throughout the order, showing its traces even in those species which have abandoned the desert habit. Containing often the only water-supply upon the desert, they are particularly liable to destruction by thirsting animals, and protection against them explains the presence of the nearly universal spines, the second marked characteristic of the order.