Dicotyledons the gamopetalous forms are admitted to be the highest
development and a dominant one of our epoch. Advance has been along
two lines, markedly in relation to insect-pollination, one of which
has culminated in the hypogynous epipetalous bicarpellate forms with
dorsiventral often large and loosely arranged flowers such as occur
in Scrophulariaceae, and the other in the epigynous bicarpellate
small-flowered families of which the Compositae represent the most
elaborate type. In the polypetalous forms progression from hypogyny
to epigyny is generally recognized, and where dorsiventrality with
insect-pollination has been established, a dominant group has been
developed as in the Leguminosae. The starting-point of the class,
however, and the position within it of apetalous families with
frequently unisexual flowers, have provoked much discussion. In
Monocotyledons a similar advance from hypogyny to epigyny is observed,
and from the dorsiventral to the radial type of flower. In this
connexion it is noteworthy that so many of the higher forms are
adapted as bulbous geophytes, or as aerophytes to special xerophilous
conditions. The Gramineae offer a prominent example of a dominant
self-pollinated or wind-pollinated family, and this may find
explanation in a multiplicity of factors.

Though best known for his artificial (or sexual) system, Linnaeus
was impressed with the importance of elaborating a natural system of
arrangement in which plants should be arranged according to their true
affinities. In his _Philosophia Botanica_ (1751) Linnaeus grouped the
genera then known into sixty-seven orders (_fragmenta_), all except
five of which are Angiosperms. He gave names to these but did not
characterize them or attempt to arrange them in larger groups.
Some represent natural groups and had in several cases been already
recognized by Ray and others, but the majority are, in the light