What are the divisions of angiosperms

The angiosperms (bedecktsamer) are the best-known and best-studied taxon in the plant kingdom. The most reliable evidence of a monophyletic origin of this group is the uniform structure of the embryo sac and, related to this, a special fertilization mode, double fertilization. The angiosperms belong to two differently sized but relatively clearly demarcated classes: the dicotyledons or Magnoliopsida and the monocotyledons or Liliopsida.

The first fossil finds come from the end of the Lower Cretaceous, so they are around 130 million years old. Unfortunately, apart from remains of leaves, stems and wood, and pollen, only a few fossils have survived.

This is especially true for flowers, because the evolution of this group of plants is essentially the evolution of the flower and the mutual adaptation of flowers and their pollinators (mostly insects). Pollen is homologous to the microspores of the heterosporous ferns (especially the seed ferns), whose surface sculpture already shows numerous pollen-typical features. The most important angiosperm characteristics have been discussed in several previous chapters. The following list can serve as a summary (based on O. ROHWEDER and P. K. ENDRESS, 1983):

  1. In addition to woody types there are herbaceous species.
  2. The conductive tissue is more differentiated than in the gymnosperms and pteridosperms. In addition to tracheids, there are (mostly) also tracheas. Their side walls are covered with ladder-shaped pits or small (not large!) Courtyard pits. The sieve tubes are associated with escort cells.

  3. The leaves are more differentiated than those of the gymnosperms. They are divided into leaf blade, stem and leaf base with or without stipules. The nerve is not dichotomous, but pinnate or parallel-veined.

  4. The vegetation point is divided into tunic and corpus. The tunic is usually multi-layered.
  5. Strong differentiation of the flowers in connection with the pollen transfer by pollinators. The flowers are mostly hermaphrodite and often secrete nectar. The flower cover consists of a calyx as a covering organ and a crown as a "display device" (optical attraction for pollinators). The ovules are surrounded by carpels (carpels), so they are protected from predators, which also include many pollinators (the original pollinators were probably beetles that fed on parts of flowers). The entirety of the carpels is called the gynoeceum. It can consist of one or more carpels, these can be free or overgrown. A pistil, in turn, is subdivided into ovary, style and stigma. The ovary contains the ovule; the scar is an organ for collecting the pollen, it also enables it to germinate. The stylus contains a conductive tissue through which the pollen tubes (germinated pollen grains) reach the egg cells enclosed in the ovary.

  6. The stamina is divided into filament and anther, an anther in turn contains two counters connected by a connective. Each theka contains two pollen sacks.

  7. The exine of the pollen grains is tectat-columellat. The ovules are provided with two integuments in the more original forms.
  8. The gametophytes are even more reduced than in the gymnosperms. The microgametophyte is only three-celled. The megagametophyte (embryo sac) is usually seven-celled (eight-nucleus). There are no archegonies.

  9. Most of the time, the embryo is cellular (not nuclear) from the start.
  10. There is a secondary endosperm that arises in connection with double fertilization; mostly it is triploid.

It is largely clear how a species is to be delimited; it is usually not difficult to assign them to a genus and family. The classification in taxa of higher rank is more problematic. Already in the last century A. ENGLER suggested the following division of the dicotyledons into subgroups due to differences in flower structure:

Monochlamydeae or Apetalae (Perianth is absent or simple or inconspicuous).

Dialypetalae (perianth present, clearly divided into calyx and crown, the petals are always free, never fused).

Sympetalae (perianth also divided into calyx and crown, but the crown is fused with leaves).

Each of these subgroups has been assigned a series of orders that largely coincide with those that are still used today and that are dealt with in the following chapters. In 1966 A. CRONQUIST (New York), A. TAKHTAJAN (St. Petersburg) and W. ZIMMERMANN (Tübingen) proposed a reorganization of the angiosperms. The angiosperms were called Magnoliophyta, so they would represent a division. Nowadays it has been abandoned again and classified as a subdivision (Magnoliophytina) of the seed plants (Spermatophyta). The two classes dicotyledons (= Magnoliopsida) and monocotyledons (Liliopsida) belong to it. The dicots, in turn, can be divided into six subclasses, the monocots into five subclasses. A decisive criterion for the subdivision of the dicotyledons is the structure of the androeceum; in other words, the number and arrangement of the stamina in the flower.

In the Magnoliidae they are often arranged in a spiral (spiral), in the Dilleniidae and Rosidae they are (mostly) in groups or in one or two whorls. But the group arrangement in finished flowers is seldom so clearly recognizable as in selected examples.

One is therefore dependent on following the development of the stamina system in the course of ontogeny. Only then can it be understood that in some Dilleniidae and Rosidae only a few stem systems are initially formed and that in the course of further development a split in the primary systems occurs. These additional plants appear on the outside of the primary plants in the Dilleniidae. One speaks therefore of a centrifugal direction of development of the stamina. With the Rosidae the situation is exactly the opposite. Here new systems arise on the inside of the first one, hence the name centripetal origin. The centrifugal and centripetal arrangements are presumably parallel developments, the helical one an original one.

The system described in the following topics is based on the structure of A. CRONQUIST (1981: "An integrated system of classification of flowering plants"). Like any other, this is also controversial.

There have always been so-called problematic taxa (species, families) that did not fit into any taxon of a higher rank. R. DAHLGREN therefore divides the angiosperms into a large number of groups (e.g. in the category between order and subclass). F. EHRENDORFER (1983) and D. FROHNE and U. JENSEN (1985) subdivide the Asteridae ("in the broader sense", senso lato [s.l.]) into new groups: Asteridae ("in the narrower sense": senso stricto [s.str.] and Lamiidae. The Lamiidae largely correspond to ENGLER's order of Tubiflorae, and the Asteridae (s. str.) that of Synandrae. For the Cornidae, FROHNE and JENSEN combine some orders that are distributed in the CRONQUIST system to the Rosidae, Dilleniidae and the Asteridae (more on this in the topic Asteridae).

The whole base of angiosperms can be compared to a "black box". From a geneticist's point of view, this means that the genes for the majority of the angiosperm traits had to be present at a very early point in time and that only different combinations of genes have accumulated in the individual branches, which - presumably through chromosome and other genome restructuring - have accumulated in different ways have been recombined. Our knowledge of the angiosperm evolution thus resembles a situation faced by a chess player - when he is shown a constellation of pieces in the middle of a game - who of course knows the rules exactly, but who is not able to extrapolate back and state how they are given constellation came about.

The above scheme suggests that in addition to the "thick branches" shown, there must also be a "stick rash", ie "thin" branches extending from the base that would represent the families, genera or species that are difficult or impossible to classify.

Since the conception of botany on-line A lot has happened in the area of ​​taxonomy and the assignment of families to parent taxa. In the last few years several - not always consistent - proposals on this subject have appeared. A high priority is now attached to the sequence analyzes on the heavy subunit of ribulose-1,5-bisphosphate carboxylase and the ribosomal RNA. Based on these and traditional taxonomic criteria, new systems have been presented, which are dealt with in the following integrated contributions.

James L. REVEAL in: Plant Taxonomy - PBIO 250 Lecture Notes deals with nomenclature problems down to the last detail with references from the www and Thorne)

Taking into account molecular biological data, the Angiosperm Phylogeny Group proposed a regrouping of the angiosperms, in which the subclass system was abolished and new groupings above the orders were proposed. A number of families became - e.g. Currently phylogenetically not justifiable - not submitted to any order. This grouping was formed in 1998 in the Ann. Missouri Bot. Gard. 85, 531-553 published.

Classification of flowering plants

We have reworked the phylogenetic scheme into an electronic version. A copy of it became a table of contents (clickable map) in botany on-line described taxa rewritten (CRONQUIST system). The DELTA database contains descriptions of all plant families of the angiosperms