The features represent flowering plant key characters. Angiosperm phloem also differs from that of gymnsoperms by the presence of more specialized and 'enhanced' conductive cells, sieve tube elements and their associated companion cells . All structures and functions associated with the carpel are unique to flowering plants. Pollination of gymnosperms involves direct contact with the exposed ovule and movement of the gymnosperm microgametophyte through an opening in integuments of the ovule, the micropyle , to the egg. The carpel, unique to flowering plants, forces a more complex path for the angiosperm microgametophyte.
Classification is fundamental in biology.
Systems of classification are conceptual structures that reflect interpretation of available data regarding evolutionary relationships. Interpretations differ and new information is constantly emerging from many sources. Efforts to classify flowering plants is, perhaps, the most ancient activity of Science. Systems produced over the past 100 years have focused on the determination of phylogenetic or evolutionary patterns. This approach requires creation of a 'model' archaic or primitive type as a foundation from which modern groups evolved.