The _Cryptogamia_, the secretly-blossoming or flowerless plants, which were formerly but little observed, have in consequence of the careful investigations of recent times been proved to present such a great variety of forms, and such a marked difference in their coarser and finer structure, that we must distinguish no less than fourteen different classes of them; whereas the number of classes of flowering plants, or _Phanerogamia_, may be limited to four. However, these _eighteen classes of the vegetable kingdom_ can again be naturally grouped in such a manner that we are able to distinguish in all _six main divisions_ or _branches_ of the vegetable kingdom. Two of these six branches belong to the flowering, and four to the flowerless plants. The table on page 82 shows how the eighteen classes are distributed among the six branches, and how these again fall under the _sub-kingdoms_ of the vegetable kingdom. The one sub-kingdom of the _Cryptogamia_ may now be naturally divided into _two_ divisions, or sub-kingdoms, differing very essentially in their internal structure and in their external form, namely, the Thallus plants and the Prothallus plants. The group of _Thallus plants_ comprises the two large branches of Tangles, or Algæ, which live in water, and the Thread-plants, or Inophytes (Lichens and Fungi), which grow on land, upon stones, bark of trees, upon decaying bodies, etc. The group of _Prothallus plants_, on the other hand, comprises the two branches of Mosses and Ferns, containing a great variety of forms. All _Thallus plants, or Thallophytes_, can be directly recognized from the fact that the two morphological fundamental organs of all other plants, stem and leaves, cannot be distinguished in their structure. The complete body of all Algæ and of all Thread-plants is a mass composed of simple cells, which is called a _lobe_, or _thallus_. This thallus is as yet not differentiated into axial-organs (stem and root) and leaf-organs. On this account, as well as through many other peculiarities, the Thallophytes contrast strongly with all remaining plants—those comprised under the two sub-kingdoms of Prothallus plants and Flowering plants—and for this reason the two latter sub-kingdoms are frequently classed together under the name of _Stemmed plants_, or _Cormophytes_. The following table will explain the relation of these three sub-kingdoms to one another according to the two different views:—