In phylogenetic nomenclature, the Pentapetalae are a large group of eudicots that were informally referred to as the "core eudicots" in some papers on angiosperm phylogenetics. They comprise an extremely large and diverse group accounting for about 65% of the species richness of the angiosperms, with wide variability in habit, morphology, chemistry, geographic distribution, and other attributes. Classical systematics, based solely on morphological information, was not able to recognize this group. In fact, the circumscription of the Pentapetalae as a clade is based on strong evidence obtained from DNA molecular analysis data. The Pentapetalae clade is composed of the orders Berberidopsidales—including the family Aextoxicaceae—Caryophyllales, Santalales and Saxifragales, the families Dilleniaceae and Vitaceae and all members of the clades Asteridae and Rosidae. Phylogenetic analyses of complete chloroplast genome sequences have provided a reliable outline of the relationships among the major Pentapetalae lineages and also provide a framework for investigating the evolutionary processes that generated a large proportion of the diversity of extant angiosperms. In light of these phylogenetic results, the current challenge for scientists in this area of botany is to identify the characters that are unique to the superasterid and superrosid clades and those that arose in parallel in both, and then to explore their evolutionary implications.
Description Pentapetalae have a characteristic type of flower made up of whorls of five pieces, as the name of the clade suggests (from Ancient Greek, penta meaning five). The perianth is composed of a differentiated calyx and corolla. The sepals are innervated by three or more vascular bundles corresponding to the vascular system of the petiole, while the petals have only one trace. The androecium usually has twice as many pieces as the calyx and corona, i.e. is composed of 10 stamens, which are arranged in two whorls. When the number of stamens is greater than twice the number of pieces of the perianth, they are arranged in fascicles or in a centrifugal spiral. Not all plants with five petals belong to Pentapetalae though. For example, several members of the family Ranunculaceae in the order Ranunculales. Gunnerales, not Ranunculales, is the sister clade to Pentapetalae, so the shared petal number between Pentapetalae and Ranunculaceae is likely the result of convergent evolution.
Pollen grains in the Pentapetalae are characteristically tricolpate. This type of pollen grain has three or more pores within grooves called "colpos". In contrast, most other spermatophytes—that is, gymnosperms, monocots and paleodicots—have monoculcate pollen, with a single pore located in a groove called a "sulcus". The gynoecium of Pentapetalae plants is usually composed of five carpels joined together, although gynoeciums formed by three carpels are also quite common. In cases where the gynoecium is composed of only two carpels, they overlap. In general, they present "compitum", a region of the style where the stylar canals of the different carpels are united in a single cavity and in which the pollen tubes can change direction of growth from one carpel to another. The ovules are usually of axillary placentation. The pistil, finally, commonly terminates in a style and a stigma that is not decurrent. The fruit is dry and dehiscent, when it is a capsule it shows loculicidal dehiscence. Regarding the interaction between pollen and pistil, pentapetalous plants have a gametophytic incompatibility system based on the RNAase system. Another anatomical characteristic of Pentapetalae is the presence of a closed root apical meristem. From the phytochemical point of view, this group of plants present cyanogenesis—that is, they biosynthesize cyanogenetic glycosides that by hydrolysis originate cyanide—through the metabolic pathway of branched amino acids, such as leucine, isoleucine and valine.
Diversity
Dilleniales
Dilleniales are recognized by their leaves with usually strong and parallel secondary veins that go straight to the teeth; being common the tertiary scalariform venation. The leaf lamina is usually rough. Also, the leaves tend to elongate when still rolled. The wood is usually vivid brown. The peduncles are jointed near the apex and persist after the flower falls off; the flowers are usually conspicuous, with ruffled petals and numerous stamens that are reflexed in the bud, usually having porous anthers. The fruits are small follicles containing seeds with aril, the calyx is persistent, sometimes acrescent, and the filaments are also persistent.
Berberidopsidales
Berberidopsidales is an order formally accepted only in the most recent phylogenetic classifications of angiosperms, comprising two small families, Aextoxicaceae and Berberidopsidaceae, which together include only three genera and four species distributed in Chile and eastern Australia. The distinctive characters of the order are largely related to its anatomy, such as the presence of crystals—especially drusen—in the leaves and petioles, the vascular bundles of the petiole form a ring and the stoma of the leaf epidermis are of a particular type called "cyclocytic". The androecium has stamens with rigid filament and the seeds present endotesta. The flowers of Berberidopsis corallina do not differ in sepals and petals, but exhibit a gradual transition from small outer tepals to larger, brightly colored inner tepals. The androcecium consists of a ring of stamens and there are three carpels with parietal placentation. In contrast, Aextoxicon has unisexual and pentamerous flowers. Male flowers have a distinct calyx and corona and a haplostomous androcecium. Female flowers have the same type of perianth, but the number of pieces is more variable. Given these differences between the two genera belonging to the same family, it has been suggested that the floral development of Berberidopsidales is a "link" in the evolution of the Gunneridae flower, and that the floral morphology of Aextoxicon, with features such as the highly variable number of sepals and spirally arranged petals, is also compatible with this hypothesis. However, the position of the Berberidopsidales in the phylogenetic tree is not congruent with it.
Santalales
… excerpt ends here. Continue reading the full article.
