The growth and development of the leaf in tomato (Lycopersicon esculentum). II. Leaf ontogeny

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Abstract
Ontogenetic studies of compound leaves which develop their lateral appendages in basipetal sequence are rare. The histogenesis of the compound leaf in Lycopersicon esculentum cv. Farthest North was followed with emphasis placed on the differentiation of the major and minor venation in true leaf number 3, using whole, cleared leaflets and transverse and paradermal microtome sections. The primary vein procambium develops acropetally and continuously into the base of the 50-μ(micron)-long leaf primordium and is observed at the tip of 200-μ-long leaves. Correlated with this event, marginal meristem activity in the apical region of the peg-like leaf primordium begins to form the lamina of the terminal leaflet. External phloem is also observed in the basal region of the primordium at this stage. Slightly later, isolated stem and leaf loci of xylem initiation are observed. Secondary vein procambium is observed initially in the terminal leaflet when the leaf is 250 to 400 μ long. The subterminal lateral leaflets appear as small bulges on the differentiating rachis during this stage. A previously undescribed marginal vein differentiates basipetally from the leaflet tips and interconnects with derivatives of the secondary and minor veins. Discrete ontogenetic stages were recognized in minor veins as they developed from the upper spongy mesophyll layer in continuity with previously differentiated veins. Vein-ending formation was a random process and appeared to depend upon the maturation stage of the surrounding veins. Histological evidence was uncovered to support the concept of ‘morphological fields’ in terms of a close interplay between planes of mesophyll cell division and subsequent minor vein development.