Alexey V. F. Ch. Bobrov

Pericarp development and fruit structure in borassoid palms (Arecaceae–Coryphoideae–Borasseae)

BACKGROUND AND AIMS The Borasseae form a highly supported monophyletic clade in the Arecaceae-Coryphoideae. The fruits of Coryphoideae are small, drupaceous with specialized anatomical structure of the pericarp and berries. The large fruits of borassoid palms contain massive pyrenes, which develop from the middle zone of the mesocarp. The pericarp structure and mode of its development in Borasseae are similar to those of Eugeissona and Nypa. A developmental carpological study of borassoid palms will allow us to describe the process of pericarp development and reveal the diagnostic fruit features of borassoid palms, determine the morphogenetic fruit type in Borasseae genera, and describe similarities in fruit structure and pericarp development with other groups of palms. METHODS The pericarp anatomy was studied during development with light microscopy based on the anatomical sections of fruits of all eight Borasseae genera. KEY RESULTS The following general features of pericarp structure in Borasseae were revealed: (1) differentiation of the pericarp starts at early developmental stages; (2) the exocarp is represented by a specialized epidermis; (3) the mesocarp is extremely multilayered and is differentiated into several topographical zones - a peripheral parenchymatous zone(s) with scattered sclerenchymatous elements and vascular bundles, a middle zone (the stony pyrene comprising networks of elongated sclereids and vascular bundles) and an inner parenchymatous zone(s); (4) differentiation and growth of the pyrene tissue starts at early developmental stages and ends long before maturation of the seed; (5) the inner parenchymatous zone(s) of the mesocarp is dramatically compressed by the mature seed; (6) the endocarp (unspecialized epidermis) is not involved in pyrene formation; and (7) the spermoderm is multilayered in Hyphaeninae and obliterated in Lataniinae. CONCLUSIONS The fruits of Borasseae are pyrenaria of Latania-type. This type of pericarp differentiation is also found only in Eugeissona and Nypa. The fruits of other Coryphoideae dramatically differ from Borasseae by the pericarp anatomical structure and the mode of its development.

Fruit Development and Pericarp Structure in Nypa fruticans Wurmb (Arecaceae): A Comparison with Other Palms

Nypa fruticans is a mangrove palm with current natural coastal distribution in the Old World tropics and hydrochorous fruit dispersal. Nypa comprises its own monotypic subfamily in Arecaceae, with high or moderate support of its sister position to other palms excluding Calamoideae or even sister to all other palms including Calamoideae. Apocarpous Nypa fruits develop from a spherical female partial inflorescence into large, tightly packed capitate infructescences. Important anatomical features of Nypa fruits (drupes of the Rhapis type) are the single-layered exocarp and endocarp, while the mesocarp is thick and multilayered; its differentiation into a hypodermis and three topographic layers is initiated in the gynoecium shortly after pollination. The outer mesocarp layer transforms into complex aerenchyma at late developmental stages, the massive stone constitutes the second mesocarp layer, and the innermost multilayered parenchymatous mesocarp layer is highly compressed and partly obliterated in mature fruit as a result of seed growth. The mode of pericarp development in Nypa is nearly identical in most aspects to those described for paracarpous fruits of Eugeissona (Calamoideae) and syncarpous fruits of all genera of Borasseae (Coryphoideae). Carpological features support the close relationship of Nypoideae, Calamoideae (Eugeissona), and Coryphoideae (Borasseae). Nypa fruits show little similarity to those of Coryphoideae (excepting Borasseae), Ceroxyloideae, and Arecoideae.

Fruit Structure of Calycanthaceae (Laurales): Histology and Development

Premise of research. Fruit structure, in contrast to flower structure, has not been the subject of detailed comparative studies in many angiosperm lineages. This study focuses on fruit morphology, anatomy, and histology of the family Calycanthaceae (Laurales), including all genera: Calycanthus (including Sinocalycanthus), Chimonanthus, and Idiospermum. The results are compared with the fruit structures of all other families of Laurales. Methodology. We use standard methods of histological research, microtome sectioning, light microscopy, and scanning electron microscopy. Histology is studied in young, midstage, and mature fruits. Pivotal results. The fruiting carpels in Calycanthaceae are covered by the floral cup (receptacle) but not fused with it. In Calycanthus and Chimonanthus, the fruitlets are drupes of the Laurus type with the stone consisting of only the endocarp (one cell layer). In Idiospermum, the fruiting carpel is technically a berry. Development shows that fruitlets of Calycanthoideae have a thin mesocarp and a lignified palisade endocarp, whereas in Idiospermoideae there is a multicell-layered mesocarp with histological subzones and a parenchymatous endocarp. Conclusions. The sclerenchymatous palisade endocarp, and the mode of its development, is similar in drupes of the Laurus type of Calycanthoideae, Hernandiaceae, and Lauraceae. In contrast, the structure of the parenchymatous pericarp, which may be partly obliterated in fruiting carpels in berries of Idiospermoideae, is similar to that in some Atherospermataceae and Monimiaceae with a berry fruit type.