The first three divisions belong to the Cryptogams, and the third and fourth divisions to the Phanerogams. This arrangement has no systematic value, but is very convenient in many ways. The Cryptogams are also known as Spore-plants, since they multiply by unicellular organs (spores), and the Phanerogams in contradistinction are called Seed-plants (Spermaphyta), since they multiply by seeds, multicellular bodies, the most important part of which is the embryo (a plant in its infancy). Mosses, Ferns, and Gymnosperms are together known as Archegoniatæ, since they possess in common a female organ of distinct structure, the Archegonium. DIVISION I. THALLOPHYTA. The thallus in the simplest forms is unicellular; in the majority, however, it is built up of many cells, which in a few instances are exactly similar; but generally there is a division of labour, so that certain cells undertake certain functions and are constructed accordingly, while others have different work and corresponding structure. Vessels or similar high anatomical structures are seldom formed, and the markings on the cell-wall are with few exceptions very simple. The Myxomycetes occupy quite an isolated position; their organs of nourishment are naked masses of protoplasm (plasmodia). As regards the external form, the thallus may be entirely without special prominences (such as branches, members), but when such are present they are all essentially alike in their origin and growth, that is, disregarding the hair-structures which may be developed. A shoot of a Seaweed or of a Lichen, etc., is essentially the same as any other part of the plant; only among the highest Algæ (Characeæ, certain Siphoneæ, _Sargassum_, and certain Red Seaweeds) do we find the same differences between the various external organs of the plant body as between stem and leaf, so that they must be distinguished by these names. _Roots_ of the same structure and development as in the Seed-plants are not found, but _organs of attachment_ (rhizoids and haptera) serve partly the biological functions of the root.