Comparative leaf development of juvenile and adult Pseudopanax crassifolius

Abstract

We describe and compare leaf development in juvenile and adult shoots of Pseudopanax crassifolius, a strongly heteroblastic tree native to New Zealand. The shoot apical meristem is significantly larger in adult plants than in juvenile plants. Leaf primordia of the two forms are morphologically comparable at inception. The allometry of leaf length and width is similar in both forms up to a length of 7 mm. However, a shape index based on the relative position of maximum leaf width indicates that their morphology diverges when leaf primordia are 300 μm long. Laminae are initiated when the leaf primordium is shorter in adult shoots than in juvenile shoots. Maturation processes of the two leaf forms are similar. Cells and tissue types expand and differentiate in an acropetal direction. When leaf length, expressed as a proportion of mature leaf length, is used as a developmental index, the timing of all stages of leaf differentiation is comparable for the two leaf forms. The juvenile form is considered to be derived from the adult form by accelerated growth of the primordial leaf axis. Key words: Pseudopanax crassifolius, heteroblasty, leaf shape, development, allometry, New Zealand.