Our flowering plants are divided into two main groups, the _dicotyledons_ and the _monocotyledons_. These terms suggest that the division is based on the nature of the seed, which is really the case, but the groups are characterised by differences in other parts. Thus, the plants which produce seeds with two cotyledons may be known by the nature of the stem, which consists of a central pith, surrounded by wood arranged in one or more rings, and the whole enclosed in an outer epidermis or in a bark. These plants also bear leaves with netted veins, and the parts of the flower are usually in whorls of four or five or multiples of four or five. Those plants whose seeds have only one cotyledon may be known by the absence of a central pith and true bark in the stem, while the wood is arranged in scattered bundles instead of in a ring or rings. They have also, generally, leaves with parallel veins; and the parts of the flower are usually in threes or multiples of three. The following table shows these features at a glance:--
Embryo with two cotyledons. Embryo with one cotyledon.
Stem with central pith, wood in Stem with no central pith, no true rings or rings, and bark. bark, and wood not in rings.
Leaves with netted veins. Leaves with parallel veins.
Parts of flower usually in fours Parts of flower arranged in threes or fives. or multiples of three.
These two great divisions or _classes_ are split up into _sub-classes_, each embracing a large number of plants with common characters; and the sub-classes are again divided into _orders_, and the orders into _genera_.
The student should always endeavour to determine the order to which any flower he finds belongs; and, if possible, the genus and the species. It is certainly a pleasure to be able to call flowers by their names, but at the same time it must be remembered that a vast deal of pleasure may be gained by the study of flowers--their peculiar structure, habits and habitats--even though their names are unknown; and the student who has learnt to recognise these characters, and to discover the relationships that exist between certain flowers of different species, is certainly much more fortunate than the one who knows abundance of names with only a meagre acquaintance with the flowers themselves.