Comparative structure-function studies within a strongly dimorphic plant, Melocactus intortus (Cactaceae)

Abstract

Melocactus intortus is a species of strongly dimorphic plants whose different parts show strong correlations between structure and function. Each individual has one unbranched monopodial shoot; during the first several years of life, this grows as a photosynthetic juvenile with a transparent epidermis, numerous stomata and a system of cortical bundles that permeate the chlorophyllous outer cortex. Cortical bundles undergo secondary growth. The xylem of the stele contains wide vessel elements, vascular tracheids and fibres, resulting in a wood that is probably relatively strong and efficient at water conduction. After several years, the plant undergoes a transition to being adult and all further shoot elongation growth results in a region of body with different anatomy and which is capable of flowering, the cephalium. The epidermis is thin, lacks stomata and is soon converted to a cork cambium. The cortex is nonphotosynthetic and cortical bundles do not have secondary growth. The wood of the adult portion contains only narrow vessels and parenchyma. The juvenile and adult portions of each shoot have distinct functions as well as distinct requirements for gas exchange; water and nutrient transport; and mechanical strength. When interpreting the selective value of the various anatomical features, it is critically important to consider the specific role and requirements of the particular portion of the plant in which they occur.