Presence Of Orbicules Research Articles

Pollen morphological support for the Catesbaeeae-Chiococceae-Exostema-complex (Rubiaceae)

The pollen morphology of the Catesbaeeae-Chiococceae-Exostema complex as recently treated by Delprete (1966) was examined with LM and SEM. The group is remarkably stenopalynous; typical representatives have medium sized, 3-colpate pollen with a perforate tectum covered with microspines. The inner nexine ornamentation is pronounced and offers more variation than the sexine pattern. A typology of the inside structures is presented based on LM observations and SEM observations of sectioned grains. Orbicules are common and numerous in the Catesbaeeae and Exostema-group; for most genera of the Chiococceae confirmation is needed of orbicule presence. All orbicules observed are relatively large (1-4 μm) and spiny. Pollen and orbicule morphology proved to be a powerful tool to delimit the Catesbaeeae-Chiococceae-Exostema complex. The overall delimitation of the complex is corroborated with our pollen data. The genera Mastixiodendron and Placocarpa, however, can be excluded from the complex based on their pollen morphology. Mastixiodendron has 3-colporate, perforate pollen without microspines and the endocolpi are fused into an endocingulum. Pollen of Placocarpa is reticulate and 3-colporate with perpendicular endocolpi.

Systematic value of tapetal orbicules: a preliminary survey of the Cinchonoideae (Rubiaceae)

Sixteen species of 14 genera of 7 different tribes of the Cinchonoideae (Rubiaceae) were investigated with a scanning electron microscope to examine orbicule presence and morphological diversity. Orbicules are lacking only in Gouldia terminalis (Hook, et Arn.) Hbd. and Simira rubescens (Benth.) Bremek. ex Steyerm. Four orbicule types could be distinguished mainly on the basis of shape and ornamentation characteristics. Spiny orbicules (type I) we found only in the Catesbaeeae (sensu Delprete). Augusta longifolia (Spreng.) Rehd. has microrugulate orbicules (type II). Smooth and spherical orbicules smaller than 0.5 μm (type III) are most widespread in our sample of the Cinchonoideae. In Burttdavya nyasica Hoyle, they are irregular and folded (type IV). For each species a description is given of the orbicule and of the sexine morphology, to allow comparison. Species with spiny orbicules always have microspines on the pollen sexine. For the other orbicule types no such resemblance could be observed. Transmission electron microscope observations of four species are included to comment on ultrastructural differences between orbicule types. From this preliminary survey, we suggest that orbicule features in Cinchonoideae may be systematically useful on generic and tribal levels. Key words: Ubisch bodies, Rubiaceae, Cinchonoideae, orbicule ultrastructure, tapetum.

Ultrastructure and Development of Mesozoic Pollen: Classopollis

Because of its complexity one of the most unusual fossil pollen types is the genus Classopollis. Grains of this broadly defined Mesozoic taxon range from the late Triassic into the Cretaceous (Turonian), and include forms that are spherical with a subequatorial rimula. On the proximal pole is a trilete mark, and on the distal surface a thin area in the sporoderm termed the cryptopore. Ultrastructural studies of Classopollis have been completed on grains extracted from the pollen cone Classostrobus comptonensis collected from the Lower Cretaceous Wealden beds on the Isle of Wight, England. The sporoderm consists of clearly defined nexine and sexine components, with the mature nexine composed of approximately 20 electron dense lamellae, each about 10 nm thick. The sexine consists of four (S1–4) easily recognizable layers, with the most prominent zone formed of coarse, inwardly-tapering elements. The S2 layer is uniformly thickened, except in specialized areas (e.g., trilete, rimula, cryptopore) where it becomes thin. The remaining wall layers include spinules that ornament the surface and a uniform series of small lacunae associated with the spinule bases. The presence of orbicules and a complex system of membranes associated with the grains extracted from less mature cones provides an opportunity to trace some developmental stages in Classopollis sporoderm ontogeny, and to compare these stages with those of selected extant pollen types. The functional significance of the infrastructure in Classopollis pollen is discussed.

THE ULTRASTRUCTURE OF SCHOPFIPOLLENITES: ORBICULES AND TAPETAL MEMBRANES

Electron microscopy of in situ fossil (Pennsylvanian) pollen assignable to the taxon Schopfipollenites indicates the presence of orbicules and tapetal membranes similar to those associated with the secretory tapetum of extant plants. Orbicules are spheroidal, hollow and associated with a complex tapetal membrane system. Each membrane is three‐parted, consisting of two thin layers attached by beads of electron‐dense material. A thicker, outer layer is suggestive of the orbicule reticulum described in some angiosperm families. A comparison between the fossil orbicules and membrane systems and those of extant plants is presented.