Appressoria are also formed by some parasitic fungi, as a minute flattening of the tip of a very short branch (Erysiphe), or the swollen end of any hypha which comes in contact with the surface of the host (Piptocephalis, Syncephalis), haustoria piercing in each case the cell-wall below.
Physiologically, any cell or group of cells separated off from a hypha or unicellular fungus, and capable of itself growing out - germinating - to reproduce the fungus, is a spore; but it is evident that so wide a definition does not exclude the ordinary vegetative cells of sprouting fungi, such as yeasts, or small sclerotium like cell-aggregates of forms like Coniothecium.
Whether a spore results from the sexual union of two similar gametes (zygospore) or from the fertilization of an egg-cell by the protoplasm of a male organ (oospore); or is developed asexually as a motile (zoospore) or a quiescent body cut off from a hypha (conidium) or developed along its course (oidium or chlamydospore), or in its protoplasm (endospore), are matters of importance which have their uses in the classification and terminology of spores, though in many respects they are largely of academic interest.
A simple sporophore may be merely a single short hypha, the end of which stops growing and becomes cut off as a conidium by the formation of a septum, which then splits and allows the conidium to fall.
More generally the hypha below the septum grows forwards again, and repeats this process several times before the terminal conidium falls, and so a chain of conidia results, the oldest of which terminates the series (Erysiphe); when the primary branch has thus formed a basipetal series, branches may arise from below and again repeat this process, thus forming a tuft (Penicillium).
Or the primary hypha y first swell at its apex, and put forth a series of short peg-like branches (sterigmata) from the increased surface thus provided, each of which develops a similar basipetal chain of conidia (Aspergillus), and various combinations of these processes result in the development of numerous varieties of exquisitely branched sporophores of this type (Botrytis, Botryosporium, Verticillium, &c.).
A second type is developed as follows: the primary hypha forms a septum below its apex as before, and the terminal conidium, thus abstricted, puts out a branch at its apex, which starts as a mere point and rapidly swells to a second conidium; this repeats the process, and so on, so that we now have a chain of conidia developed in acropetal succession, the oldest being below, and, as in Penicillium, &c., branches put forth lower down may repeat the process (Hormodendron).
In Mucor, for example, the end of the primary hypha swells into a spheroidal head (sporangium), the protoplasm of which FIG.
Infection in these cases occurs in the seedling at the place where root and shoot meet, and the infecting hypha having entered the plant goes on living in it and growing up with it as if it had no parasitic action at all.
The ordinary mycelium is the gametophyte since it hears the ascogonia and aritheridia when present; the cqe Two questions of great theoretical importance have been raised over and over again in connexion with yeasts, namely, (I) the morphological one as to whether yeasts are merely degraded forms of higher fungi, as would seem implied by their tendency to form elongated, hypha-like cells in the veils, and their development of "ascospores" as well as by the wide occurrence of yeast-like "sprouting forms" in other fungi (e.g.
They present a hypha-like appearance, running longitudinally for considerable distances.
Saprophytic bacteria can readily make their way down the dead hypha of an invading fungus, or into the punctures made by insects, and Aphides have been credited with the bacterial infection of carnations, though more recent researches by Woods go to show the correctness of his conclusion that Aphides alone are responsible for the carnation disease.