Sympodial construction of Fibonacci-type leaf rosettes in Pinguicula moranensis (Lentibulariaceae)

Abstract

Background and aims: The leaf rosettes of the carnivorous Pinguicula moranensis follow a spiral phyllotaxis approaching a Fibonacci pattern while the stalked flowers arise from extra-axillary sites between the leaves. The organization of this rosette has been discussed by various authors, with various results. The aim of the present study was to clarify the development of the flowering rosettes of this species.

Methods: The formation of the rosettes is shown with the aid of scanning electron microscopy.

Key results and conclusions: The scanning electron micrographs show that each flower terminates an article (sympodial unit). The leaves of consecutive articles of such sympodially constructed rosettes are arranged along a spiral Fibonacci pattern (with divergence angles around 137 degrees). This results from homodromy of leaf initiation in consecutive articles with the first leaf (prophyll) of a new article inserted in an obliquely transverse position next to the floral scape that terminates the former article. Sympodial construction of flowering shoots and leaf rosettes is also known from Aloe, Gunnera and Philodendron. As a by-product of this study, the unidirectional development of the Pinguicula flower is confirmed and discussed.

Fig. 1.

Pinguicula moranensis. (A) Flowering individual, dividing vegetatively into daughter rosettes. Each rosette has one, two or three flowers on long stalks (scapes). In one flower, the flag petals are marked with asterisks. Scale bar = 40 mm. (B) Top view of rosette. The observable leaves are labelled along the ontogenetic spiral (also called genetic spiral, i.e. according to their initiation order) from the centre to the periphery. Note Fibonacci pattern with contact parastichies connecting leaves with age differences of three (solid line), five (short dashed line) and eight (long dashed line) plastochrones. The positions of three flowers F_I, F_II and F_III are shown by their scape bases (flowers and upper scape portions removed). Scale bar = 15 mm. (C, D) Centre of dissected rosette, lateral and top view. Flower bud on a stalk (young scape). Calyx lobes show descending cochlear aestivation, with two outer lobes (K_o) covering three inner lobes (K_i). (C) All leaves except leaf primordia 1–3 partly or totally removed. (D) Leaves 1–4 (surrounded by dashed line) belong to a sympodial unit (new article), which is subtended by leaf 5 (S = subtending leaf). Former article (with leaves 5–7) is terminated by the flower bud. Scale bars = 0·5 mm.

Fig. 2.

Pinguicula moranensis. (A, B) Centre of another dissected rosette with flower bud; (A) overview and (B) close-up. All leaves removed, except leaf primordia 1–3. Leaves 1–3 (surrounded by dashed line) belong to a sympodial unit (new article), which is subtended by leaf 4 (S = subtending leaf). Former article (with leaves 4–9) is terminated by flower (F). Two divergence angles (both around 137°) are marked for illustration with curved double arrows: between leaves 4 and 5 (within article) and between leaves 3 and 4 (across articles). Note primordial floral organs: K_o, outer sepals; K_i, inner sepals; P, five petals, with two lobes forming upper lip (i.e. flag petals) marked with asterisks; St, two primordial stamens; G, primordial gynoecium. Scale bars: (A) 0·5 mm, (B) 0·25 mm. (C, D) Centre of another rosette with a dissected flower bud; (C) overview and (D) close-up. All leaves removed, except leaf primordia 1–4. Leaves 1–4 belong to a new sympodial unit (i.e. youngest article), which is subtended by leaf 5 (S_III). Next older article (with leaves 5–8, surrounded by dashed line) is terminated by flower (F_III) and subtended itself by leaf 9 (S_II) of the previous article. This article is highlighted by a solid line and consists of two leaves (9 and 10) and a terminal flower (F_II, removed). Oldest observable article terminates with flower (F_I, again removed) after formation of leaves 11 and 12. Leaf 11 acts as subtending leaf (S_I) for the next inner article. Arrowhead points to an additional scaly leaf at the base of scape (F_III). Other abbreviations as above. Scale bars = 0·25 mm.