HYPHOMYCETES.--The predominant feature in the structure of this order has already been intimated to consist in the development of the vegetative system under the form of simple or branched threads, on which the fruit is generated. The common name of mould is applied to them perhaps more generally than to other groups, although the term is too vague, and has been too vaguely applied to be of much service in giving an idea of the characteristics of this order. Leaving the smaller groups, and confining ourselves to the _Dematiei_ and the _Mucedines_, we shall obtain some notion of the prevalent structure. In the former the threads are more or less carbonized, in the latter nearly colourless. One of the largest genera in _Dematiei_ is _Helminthosporium_. It appears on decaying herbaceous plants, and on old wood, forming effused black velvety patches. The mycelium, of coloured jointed threads, overlays and penetrates the matrix; from this arise erect, rigid, and usually jointed threads, of a dark brown, nearly black colour at the base, but paler towards the apex. In most cases these threads have an externally cortical layer, which imparts rigidity; usually from the apex, but sometimes laterally, the spores are produced. Although sometimes colourless, these are most commonly of some shade of brown, more or less elongated, and divided transversely by few or many septa. In _Helminthosporium Smithii_, the spores much exceed the dimensions of the threads;[m] in other species they are smaller. In _Dendryphium_, the threads and spores are very similar, except that the threads are branched at their apex, and the spores are often produced one at the end of another in a short chain.[n] In _Septosporium_ again, the threads and spores are similar, but the spores are pedicellate, and attached at or near the base; whilst in _Acrothecium_, with similar threads and spores, the latter are clustered together at the apex of the threads. In _Triposporium_, the threads are similar, but the spores are tri-radiate; and in _Helicoma_, the spores are twisted spirally. Thus, we might pass through all the genera to illustrate this chief feature of coloured, septate, rather rigid, and mostly erect threads, bearing at some point spores, which in most instances are elongated, coloured, and septate. [Illustration: FIG. 26.--_Acrothecium simplex._] [Illustration: FIG. 27.--_Peronospora Arenariæ._]