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

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Abstract
Plants of Asclepias perennis Watt. and A. tuberosa L. ssp. folfsii 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 pallisade 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 occurs in the leaves and stems of both species and also are found in the roots of A. tuberosa from dry, sunny and wet, shaded sites. Primary growth in the herbaceous stems of both species involves a bicollateral bundle stage and a cambium-like meristem stage. Secondary growth in the stems of A. perennis and the herbaceous stems of A. tuberosa involves cambial activity. A foraminate pattern of anomalous secondary growth occurs throughout the caudex of A. tuberosa, although an eccentric pattern occurs when a herbaceous stem develops from this organ. Asclepias perennis has slender adventitious roots, whereas A. tuberosa has a thick taproot. Distinctive hypodermal cells occur in the roots of both species, and druse-containing cells are noted in the roots of A. tuberosa. Although cambial activity is observed in the roots of both species, the taproots of A. tuberosa also exhibit a four-armed cross and foraminate type of anomalous secondary growth.