The caryophyllids are one of the major subgroups of eudicot flowering plants with more than 11,000 species. The group was one of the first recognized higher-level taxa within the angiosperms, even though it was not formally named and recognized until the 1860s. Though the group is well-defined and easily recognized, relationships among the various groups is not certain, and neither is it entirely clear how the caryophyllids are related to other groups of flowering plants. The cladogram above is an amalgamation based on several recent studies; there is still no true consensus on relationships within the caryophyllids.
In the cladogram above, the taxa along the top and right side (Chenopodiaceae around to Portulacaceae) belong to the order Caryophyllales, perhaps the most easily defined order among all plants because of its numerous synapomorphies. One of the first chacteristics recognized as common within the group was the placentation, or arrangement and attachment of ovules (immature seeds) within the ovary (immature fruit). Placentation in the Caryophyllales is often basal or free central, meaning that the ovules are all attached to the bottom floor of the ovary chamber, or are attached to a central pillar within the ovary. Thus when the fruit matures from the ovary, all the seeds are located in the center of the fruit, from which the group gets one of its other names: Centrospermae, or "central seeds".
A second set of important characteristics may be found within the seeds of the Caryophyllales. The embryo curls around the outer egde of the seed, rather than lying in the center, and the nutritive tissue in the center of the seed is diploid perisperm rather than triploid endosperm as in other flowering plants.
Other characters for the group are a bit more complicated. A third character lies in the chemical composition of certain cell walls; these cells have ferulic acid in their walls, which is not found in any other groups of dicots. A fourth character is the plastid structure in the phloem cells; the Caryophyllales have P-type plastids in contrast to the S-type found in other dicots. Finally, all Caryophyllales (with the exception of the Caryophyllaceae and the smaller family Molluginaceae) have a unique class of chemical pigements called betalains. These pigments have been found in no other plants, and the anthocyanin pigments that are found in other plants are absent in the betalain-producing groups.
Botanists have long hypothesized that the closest relatives of the Caryophyllales were the Polygonaceae and Plumbaginaceae, based on similarities in seed morphology and largely for intuitive reasons. Recent phylogenetic analyses using morphological and molecular data support a close relationship of these families with the Caryophyllales.
One recent surprise resulting from these molecular studies is the inclusion of Droseraceae (sundew family) and Nepenthes (Asian pitcher plants) with the caryophyllids. These groups are both carnivorous, and so are highly unusual plants that have been difficult to classify, but no one had previously suspected that they might be related to carnations and cactus!
Likewise, no one expected the Dilleniaceae to be a close relative of this group, but molecular studies have tenatively included that family as a basal branch in the caryophyllid tree.
For more information on current thought in caryophyllid systematics, visit the Tree of Life.