Lariales or Diatoms

LARIALES or DIATOMS, a group of minute, brown plants sometimes classed with the Algae; and the FUNGI and BACTERIA, which are colourless parasitic or saprophytic plants probably derived by reduction from alga-like ancestors. To all these groups special articles are devoted, and also to the lichens, which are a peculiar group of composite plants, consisting of an alga and a fungus living in association.

The BRYOPHYTA (q.v.), which include the liverworts and the Musci or true mosses, are distinguished from the Thallophyta by the fact that the plant body often shows a distinction into stem and leaves, and the female reproductive organs are structures known as archegonia.

The group PTERIDOPHYTA, or fern-like plants, includes all the cryptogams higher than the Bryophyta. The plants often show a marked differentiation into leaves, some of these being of very, great size and much divided. Furthermore they show well marked vascular strands (see p. 6) and their female reproductive organs are archegonia. This large group includes the Filicineae (true ferns), the Equisetineae (horsetails, etc.) ; the Lycopodineae (club-mosses, etc.) and the Pteridospermae (a fossil group) which are described under PALAEOBOTANY.

Phanerogams.

To this group, which is sometimes termed SPERMOPHYTA (or Spermatophyta), all the higher plants belong. It includes the GYMNOSPERMS, in which the seed is not enclosed in an ovary and the male and female reproductive organs are not found contained in a single flower, and the ANGIOSPERMS, in which the seed is not naked but enclosed in an ovary and the male and female reproductive organs (stamens and carpels) are usually associated in a single flower.

Classification of Angiosperms.—This very large group of plants falls into two classes, MONOCOTYLEDONS and DICOTYLEDONS. The first being characterised by an embryo with one cotyledon (seed leaf), a stem which usually shows no secondary thickening, and flowers with parts in threes. The second shows an embryo with two cotyledons, a stem which exhibits secondary thickening, and flowers with parts in fours or fives. The classification of angio sperms which is now usually followed, is that of A. Engler, the monocotyledons and dicotyledons being divided into a number of classes termed orders (or cohorts or series), the name of each order usually terminating in -ales. Each order includes a number of families (formerly termed "natural orders") many of which are dealt with in special articles; the name of the family usually terminates in -aceae.

An outline of Engler's classification is given below, the orders (series) being printed in block type and the families in italics. For the II orders of Monocotyledons a synopsis of the chief characteristics is given. A selection of the more important families is made in the case of the orders which include a large number of families.