The trend of the evolution of the plant kingdom has been in the direction of the establishment of a vegetation of fixed habit and adapted to the vicissitudes of a life on land, and the Angiosperms are the highest expression of this evolution and constitute the dominant vegetation of the earth's surface at the present epoch.
The Gymnosperms, with the Angiosperms, constitute the existing groups of seed-bearing plants or Phanerogams: the importance of the seed as a distinguishing feature in the plant kingdom may be emphasized by the use of the designation Spermophyta for these two groups, in contrast to the Pteridophyta and Bryophyta in which true seeds are unknown.
The flower is a characteristic feature of the highest group of the plant kingdom - the flowering plants (Phanerogams) - and is the name given to the association of organs, more or less leaf-like in form, which are concerned with the production of the fruit or seed.
Thallophyta.The simplest members of both the Algae and the Fungi (q.v.) (the two divisions of the Thailophyta, which is the lowest of the four great groups into which the plant-kingdom is divided) have their bodies each composed of a single cell.