LEAF HISTOGENESIS IN LACTUCA SATIVA WITH EMPHASIS UPON LATICIFER ONTOGENY

The leaf primoidia of Lactutca sativa 'Meikoningen' develop from a subapical initial in the second layer of the tunica on the side of a flat shoot apex. Subsequent growth of the subsurface lamina is initiated by submarginal initials which divide anticlinally to produce an adaxial layer and abiseriate abaxial layer, and periclinally to produce a middle layer from which procambium differentiates. The protoderm is derived from the first tunica layer by continuous anticlinal divisions. The activity of the subapical and submarginal initials is completed when the leaf is 0.3 mm in length and 4.0 mm in width, respectively. Continued growth of the leaf to 130-150 mm results from intercalary cell division and enlargement. The mature venation is visibly delineated when the leaf is 25-30 mm in length. Laticifer and phloem cells are initiated by the same mother cells in the procambium. The former become non-septate laticifers by resorption of cross walls. They mature concurrently with the phloem and before the xylem. THERE ARE numerous reports concerning various aspects of leaf histogenesis in dicotyledonous plants. The work on foliar venation has been reviewed by Foster (1952), and leaf histogenesis and vascular differentiation by Esau (1965a, b). The latex-containing elements of plants are classified as articulated or non-articulated laticifers. The development of the articulated laticifer, which is the type found in Lactuca sativa, has been studied in the embryo, root, stem and apical meristem in several diverse genera (Scott, 1882, 1884, 1886; Sperlich, 1939; Krotkov, 1945). Descriptions of laticifer structure and leaf development in Lactuca sativa are explained by Hayward (1938). This investigation was undertaken to follow the development of the lamina of Lactuca sativa with emphasis on the ontogeny of the laticifers. The results contribute to an explanation of a physiological breakdown in lettuce (tipburn) resulting from abnormal laticifer development and subsequent rupture in rapidly enlarging leaves (Tibbitts, Struckmeyer, and Rao, 1965; Olson, Tibbitts, and Struckmeyer, 1967). MATERIALS AND METHODS-A small butterhead cultivar of lettuce ,'Meikoningen,' was used in this study. Seeds were planted in well-fertilized peat 1 Received for publication 15 April 1969. This work represents a portion of a dissertation in partial fulfillment of the requirements for the Ph.D. degree in the Department of HorticultuLre, University of Wisconsin. Appreciation is expressed to Dr. Ray Evert for his counsel during the course of this investigation. Published with the permission of the Director, Research Programs Division, College of Agricultural and Life Sciences, University of Wisconsin, Madison 53706. 2 Present address: Biology Department, Hiram Scott College, Scottsbluff, Nebraska 69361. soil in gallon containers and placed in a growth chamber with a non-barrier type light bank of both fluorescent and incandescent light at approximately 1,800 ft-c for 16 hr daily. The temperature was maintained at 70 F ?i 2 F day and night and the relative humidity above 80%7. Parts of leaves sampled at different stages of development were prepared for study by paraffiin sectioning and by tissue clearing. The samples for sectioning were killed in FPA, dehydrated with n-butyl alcohol, and infiltrated and embedded in Tissuemat. Paradermal and transverse sections were cut 6-10 , thick and stained with safranin and fast green FCF (Sass, 1958). Two clearing techniques were employed which provided excellent, precise methods for viewing the different layers within the leaf. The lactic acid method of Simpson (1929) was rapid and effective. Immersion of the entire leaf in 75 % lactic acid for 2 to 3 days at 54 C was found most effective. These leaves were not stained. Other leaves were killed in FAA, treated with NaOH and chloral hydrate, stained with safranin and mounted in Harleco's synthetic resin (Brady, Wemple, and Lersten, 1965). RESULTS Initiation and development of leaf primordia-Leaf primordia are initiated on the flanks of a broad, flat shoot apex (Fig. 1). In the second tunica layer a cell becomes a subapical initial. The subapical initial divides periclinally, the outer derivative remaining a subapical initial and the inner one giving rise to the middle tier of cells of the primordium (Fig. 2). The first and second tunica layer grow with the developing primordium by frequent anticlinal divisions. Subsequently, cells of the second tunica layer divide periclinally, adding to the width of the pri