Resembling the smaller club mosses in their development, but differing in some important points, are the quill-worts (_Isoeteæ_). They are mostly aquatic forms, growing partially or completely submerged, and look like grasses or rushes. They vary from a few centimetres to half a metre in height. The stem is very short, and the long cylindrical leaves closely crowded together. The leaves which are narrow above are widely expanded and overlapping at the base. The spores are of two kinds, as in _Selaginella_, but the macrosporangia contain numerous macrospores. The very large sporangia (_M_, _sp._) are in cavities at the bases of the leaves, and above each sporangium is a little pointed outgrowth (ligula), which is also found in the leaves of _Selaginella_. The quill-worts are not common plants, and owing to their habits of growth and resemblance to other plants, are likely to be overlooked unless careful search is made. CHAPTER XIV. SUB-KINGDOM VI. SPERMAPHYTES: PHÆNOGAMS. The last and highest great division of the vegetable kingdom has been named _Spermaphyta_, "seed plants," from the fact that the structures known as seeds are peculiar to them. They are also commonly called flowering plants, though this name might be also appropriately given to certain of the higher pteridophytes. In the seed plants the macrosporangia remain attached to the parent plant, in nearly all cases, until the archegonia are fertilized and the embryo plant formed. The outer walls of the sporangium now become hard, and the whole falls off as a seed. In the higher spermaphytes the spore-bearing leaves (sporophylls) become much modified, and receive special names, those bearing the microspores being commonly known as stamens; those bearing the macrospores, carpels or carpophylls. The macrosporangia are also ordinarily known as "ovules," a name given before it was known that these were the same as the macrosporangia of the higher pteridophytes. In addition to the spore-bearing leaves, those surrounding them may be much changed in form and brilliantly colored, forming, with the enclosed sporophylls, the "flower" of the higher spermaphytes. As might be expected, the tissues of the higher spermaphytes are the most highly developed of all plants, though some of them are very simple. The plants vary extremely in size, the smallest being little floating plants, less than a millimetre in diameter, while others are gigantic trees, a hundred metres and more in height. There are two classes of the spermaphytes: I., the Gymnosperms, or naked-seeded ones, in which the ovules (macrosporangia) are borne upon open carpophylls; and II., Angiosperms, covered-seeded plants, in which the carpophylls form a closed cavity (ovary) containing the ovules. CLASS I.--GYMNOSPERMS (_Gymnospermæ_).