=Vegetative Organs.= The vegetative parts of a Fungus are termed its _mycelium_.[10] This is formed of a mass of long, cylindrical, branched cells resembling threads (and hence termed _hyphæ_), which have a continued apical growth. The mycelium, in its early development, shows a well-marked difference between the two main groups of true Fungi: in the _Phycomycetes_, or Algal Fungi, the mycelium has no transverse walls, and is therefore unicellular, while in the _Mesomycetes_ and _Mycomycetes_ it is provided with dividing walls, which gradually arise during growth, in the youngest hyphæ; intercalary transverse walls may also be formed at a later period. In the hyphæ of some of the Higher Fungi (_Hymenomycetes_), connections may be formed between two contiguous cells of the same hypha, by a protuberance growing out from an upper cell just above the transverse wall, and forming a junction with the cell below. These are known as _clamp-connections_; they appear to be of use in affording communication between the two cells. The hyphæ of Fungi, where they come in contact with one another, often grow together, so that =H=-formed combinations (fusions) are produced, which give rise to very compact felted tissue. When the hyphæ are not only closely interwoven, but also united and provided with many transverse walls, the mycelium assumes the appearance of a tissue with isodiametric cells, and is then termed _pseudo-parenchyma_. The hyphæ-walls are sometimes very much thickened, and composed of several layers, and the external layers, by the absorption of water, may often swell very much and become mucilaginous. In some instances the walls are colourless, in others coloured, the most frequent colour being brown. The cell-contents may also be coloured, and in that case are generally yellow; this colour is chiefly connected with the fat (oil) which may be found in abundance in the Fungi, whilst starch is invariably absent in all the true Fungi. The mycelium assumes many different forms; sometimes it appears as a thread-like, cobwebby, loose tissue, less frequently as firm strands, thin or thick membranes, horn-like plates or tuber-like bodies. The _thread-like_ mycelium may, in the parasitic Fungi, be intercellular or intracellular, according as it only extends into the interstices between the cells or enters into the cells proper. In the first case there are generally found haustoria, or organs of suction (_e.g._ among the _Peronosporaceæ_; _Taphrina_, on the contrary, has no haustoria); but haustoria are also found among the epiphytic Fungi (_e.g._ Erysiphaceæ). Intracellular mycelia are found in the Rust-Fungi, in _Claviceps purpurea_, _Entomophthora_, etc. In spite of its delicate structure, this mycelium may live a long time, owing to the circumstance that it continues to grow peripherally, while the older parts gradually die off (“fairy rings”).