Various modifications of these indehiscent fruits are to be met with; thus, the blackberry is not really a berry, but a cluster of little drupes formed from a single pistil of many carpels. A berry, too, may be made up of many parts, as is the case with the orange. The apple and similar fruits consist of a core (the true fruit) surrounded by a fleshy mass that is produced from the receptacle of the flower; and the strawberry is a succulent, enlarged receptacle of the flower, with a number of little achenes (the true fruits) on its surface. The seed, as we have already observed, is the embryo plant. It consists of one or more seed-leaves or _cotyledons_, a _radicle_ or young root, and a _plumule_ or young bud. In many cases the skin of the seed encloses nothing more than the three parts of the embryo, as named above; but it sometimes contains, in addition, a quantity of nutrient matter in the form of albumen, starch, oil, gum, or other substance. CLASSIFICATION OF FLOWERING PLANTS 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:-- _Dicotyledons_ _Monocotyledons_ 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_.