Comparative Anatomy and Morphology of Asclepias perennis and Asclepias tuberosa Subspecies Rolfsii

Abstract

Plants of Asclepias perennis Watt. and A. tuberosa L. ssp. rolfsii are examined to determine structural characteristics of these species from different habitats. Some structural features in both species are related to growth habit. Stomata occur more frequently on the adaxial surface of the leaves of both species from the dry, sunny sites and are more abundant on the abaxial surface in leaves of A. perennis from these sites. The dorsiventral leaves have well-developed palisade and spongy mesophyll layers and are thicker in plants from the dry, sunny sites because of a large palisade mesophyll. Leaves of both species have greater length-to-width ratios in plants from the dry, sunny sites than from wet, shaded sites. Bicollateral bundles constitute the midrib and major veins of the leaves and petiole bundles, while collateral bundles constitute the minor veins of the leaves. Druse-containing cells and branched, nonarticulated laticifers occur in the leaves and stems of both species and also are found in the...