Part 161. The seed-plant division (Spermatophyta) is coextensive with that branch of the Vegetable Kingdom commonly known as Phanerogamia, phenogams, or flowering plants, because characterized by the production of flowers containing at least either pollen-sacs or ovules. Since the production of seed is the function of these parts, and since no other plants produce true seeds containing an embryo, it is equally appropriate to speak of them as seed-plants, seedworts, or spermatophytes.

The system of classification (although not always the sequence of groups) adopted in the foregoing pages is substantially that of Engler and Prantl whose great work on the natural families of plants is now most generally followed, at least, with regard to phenogams. In this classification there are recognized among seed-plants about fifty orders and two hundred and eighty families.

The eighteen orders, thirty-two families, and about a hundred genera of seed-plants included in this chapter are represented by formulas on pages 404-427 in order that the student may readily compare the more important structural characters of one group with those of another, and so gain a better grasp of the abstract ideas underlying a natural classification. Taken in connection with the accounts of the various groups given in the sections referred to by number before each formula, and with reference to the figures indicated in each section, the formulas will afford a most profitable means of reviewing the many details already studied, and will reveal some of their wider relations.