Traditionally, the flowering plants have been divided into two major groups, or classes: the dicots (Magnoliopsida) and the monocots (Liliopsida). Many people take this separation into two classes for granted, because it is "plainly obvious", but even after their general acceptance, botanists did not always agree upon the placement of families into one or the other class. Even in this century some plants called paleoherbs have left problems for taxonomy of angiosperms. These plants have a mix of characters which do not occur together in most other flowering plants and confound strict classification (e.g. the Nymphaeales, or water lilies). There are also monocots which posses characters more typical of dicots e.g. the Dioscoreales, Smilacaceae, Alismataceae and Potamogeton).
This "fuzziness" in the definitions of monocots and dicots is not simply the result of poor botany. Rather, it is a real phenomenon resulting from the shared ancestry of the two groups. It is now believed that some of the dicots are more closely related to monocots than to the other dicots, and that the angiosperms do not all fit neatly into two classes.
The 'Cronquist System' of flowering plant classification groups flowering plants (Magnoliophyta) into two classes (monocots and dicots) with related orders (groups of families) placed in subclasses. (see Arthur Cronquist, 1988 The Evolution and Classification of Flowering Plants).
The 'Takhtajan System' of flowering plant classification treats flowering plants as a division or phylum (Magnoliophyta) with two classes (monocots and dicots) which are organized into subclasses. Higher level organization is similar to the Cronquist System, but a bit more complex. Takhtajan uses the superorder as the basic unit of the subclass and this pattern of organization is also used in the Thorne System of flowering plant classification (see Armen Takhtajan, 1997, Diversity and Classification of Flowering Plants).
Thorne treats the flowering plants at the rank of class with an initial bifurcation into two subclasses. Related orders are placed within each subclass as superorders and, within these, clusters of related families are grouped into suborders. There is detailed classification (subfamilies, tribes) for larger families.
In Europe conservationists estimate that some 1,000 flowering plants are in danger, of which only 244 are officially recognized as endangered.