MORPHOLOGY AND VASCULATURE OF THE LEAF OF POTATO (SOLANUM TUBEROSUM)

The leaf and stem of the potato plant (Solanum tuberosum L. cv. Russet Burbank) were studied by light microscopy to determine their morphology and vasculature; scanning electron microscopy provided supplemental information on the leaf's morphology. The morphology of the basal leaves of the potato shoot is quite variable, ranging from simple to pinnately compound. The upper leaves of the shoot are more uniform, being odd pinnate with three major pairs of lateral leaflets and a number of folioles. The primary vascular system of the stem is comprised of six bundles, three large and three small ones. The three large bundles form a highly interconnected system through a repeated series of branchings and arch-producing mergers. Two of the three large bundles give rise to short, lateral leaf traces at each node. Each of the small bundles in the stem is actually a median leaf trace which extends three intemodes before diverging into a leaf. The three leaf traces enter the petiole through a single gap; thus the nodel anatomy is three-trace unilacunar. Upon entering the petiole, each of the laterals splits into an upper and a lower lateral. Whereas the upper laterals diverge entirely into the first pair of leaflets, the lower laterals feed all of the lateral leaflets through a series of bifurcations. Prior to their entering the terminal leaflet, the lower laterals converge on the median bundle to form a single vascular crescent which progresses acropetally into the terminal leaflet as the midvein, or primary vein. In the midrib, portions of the midvein diverge outward and continue as secondaries to the margin on either side of the lamina. Near the tip of the terminal leaflet, the midvein consists of a single vascular bundle which is a continuation of the median bundle. Six to seven orders of veins occur in the terminal leaflet. THE ANGIOSPERMOUS LEAF functions as a highly efficient organ as it manufactures photosynthates and exports them to other parts of the plant. The capacity of a leaf to export assimilates is determined in part by the extent and arrangement of its vascular system; moreover, the pathway followed by the photosynthates out of a leaf is determined in large part by the canalizing effect of the vascular organization of the leaf. Thus, mapping the details of the vasculature of a mature leaf is an indispensable prerequisite for understanding the functional aspects of assimilate transport, over both short and long distances. In recognition of the interdependence of vascular structure and function in the tightly coordinated process of solute transport, this laboratory has been involved in the description of the vasculature of mature leaves in a number of species (Fisher and Evert, ' Received for publication 10 March 1987; revision accepted 9 June 1987. This research was supported by an NSF predoctoral fellowship and a University of Wisconsin-Madison Dean's fellowship to MMM and by NSF research grants PCM8003855 and PCM-8302442 to RFE. The authors would like to thank Claudia Lipke for photographic assistance, Lucy Taylor for the illustrations of the stem vasculature, and Dr. R. R. Kowal for statistical advice. 4 Address reprint requests to this author. 1982; Colbert and Evert, 1982; Russin and Evert, 1984; Russell and Evert, 1985). This paper is the first in a series of articles describing various aspects of the vasculature of the compound leaf of the potato plant (Solanum tuberosum L. cv. Russet Burbank). The potato plant is a widely cultivated and economically important crop plant that has been studied intensively for many years. We are particularly interested in the potato plant because of the bicollateral veins which occur in its leaves (Artschwager, 1918). The presence of bundles with both adand abaxial phloem makes the study of structure/function relationships in the potato leaf both more complex and more interesting. Although a fair amount of structural information on the potato leaf already exists, these data are scattered throughout the literature and are not always readily accessible. In addition, detailed information concerning certain aspects of the leaf, such as the structure of the various vein orders, is virtually lacking. This paper will unite the previously disparate descriptions of many of the key features ofthe morphology and vasculature of the potato leaf, clarify some features, such as the stem vasculature, and describe for the first time certain others, such as the vasculature of the petiole-rachis axis. In addition, this ar-