The segments of the hyphae in this group usually contain several nuclei. At the time of sporangial formation the protoplasm with numerous nuclei streams into the swollen end of the sporangiophore and there becomes cut off by a cell-wall to form the sporangium. The protoplasm then becomes cut up by a series of clefts into a number of smaller and smaller pieces which are unicellular in Pilobolus, multicellular in Sporodinia. These then become surrounded by a cell-wall and form the spores. This mode of spore-formation is totally different from that in the ascus; hence one of the difficulties of the acceptance of Brefeld’s view of the homology of ascus and sporangium. The cytology of zygospore-formation is not known in detail; the so-called gametes which fuse are multinucleate and are no doubt of the nature of gametangia. The fate of these nuclei is doubtful, probably they fuse in pairs (fig. 6).
Blakeslee has lately made some very important observations of the Zygomycetes. It is well known that while in some forms, e.g. Spordinia, zygospores are easily obtained, in others, e.g. most species of Mucor, they are very erratic in their appearance. This has now been explained by Blakeslee, who finds that the Mucorinae can be divided into two groups, termed homothallic and heterothallic respectively. In the first group zygospores can arise by the union of branches from the same mycelium and so can be produced by the growth from a single spore; this group includes Spordinia grandis, Spinellus fusiger, some species of Mucor, &c. The majority of forms, however, fall into the heterothallic group, in which the association of branches from two mycelia different in nature is necessary for the formation of zygospores. These structures cannot then be produced from the product of a single spore nor even from the thalli derived from any two spores. The two kinds of thalli Blakeslee considers to have a differentiation of the nature of sex and he distinguishes them as (+) and (−) forms; the former being usually distinguished by a somewhat greater luxuriance of growth.
The classification of the Mucorini depends on the prevalence and characters of the conidia, and of the sporangia and zygospores—e.g. the presence or absence of a columella in the former, the formation of an investment round the latter. Most genera are saprophytes, but some—Chaetocladium, Piptocephalis—are parasites on other Mucorini, and one or two are associated casually with the rotting of tomatoes and other fruits, bulbs, &c., the fleshy parts of which are rapidly destroyed if once the hyphae gain entrance. Even more important is the question of mycosis in man and other animals, referred to species of Mucor, and investigated by Lucet and Costantin. Klebs has concluded that transpiration is the important factor in determining the formation of sporangia, while zygote-development depends on totally different conditions; these results have been called in question by Falck.
The Entomophthoraceae contain three genera, Empusa, Entomophthora and Basidiobolus. The two first genera consist of forms which are parasitic on insects. Empusa Muscae causes the well-known epidemic in house-flies during the autumn; the dead, affected flies are often found attached to the window surrounded by a white halo of conidia. B. ranarum is found in the alimentary canal of the frog and growing on its excrement. In these three genera the conidia are cast off with a jerk somewhat in the same way as the sporangium of Pilobolus.
B. Higher Fungi.—Now that Brefeld’s view of the origin of these forms from the Zygomycetes has been overthrown, the relationship of the higher and lower forms of fungi is left in obscurity. The term Eumycetes is sometimes applied to this group to distinguish them from the Phycomycetes, but as the same name is also applied to the fungi as a whole to differentiate them from the Mycetozoa and Bacteria, the term had best be dropped. The Higher Fungi fall into three groups: the Ustilaginales, of doubtful position, and the two very sharply marked groups Basidiales and Ascomycetes.
| From Vine’s Students’ Text Book of Botany, by permission of Swan Sonnenschein & Co. |
| Fig. 7.—Germinating resting-gonidia. A, of Ustilago receptaculorum; B, of Tilletia Caries. |
| sp, The gonidium. pm, The promycelium. d, The sporidia: in B the sporidia have coalesced in pairs at v. |
I. Ustilaginales.—This includes two families Ustilaginaceae (smuts) and Tilletiaceae (bunts). The bunts and smuts which damage our grain and fodder plants comprise about 400 species of internal parasites, found in all countries on herbaceous plants, and especially on Monocotyledons. They are remarkable for their dark spores developed in gall-like excrescences on the leaves, stems, &c., or in the fruits of the host. The discovery of the yeast-conidia of these fungi, and their thorough investigation by Brefeld, have thrown new lights on the group, as also have the results elucidating the nature of the ordinary dark spores—smuts, bunt, &c.—which by their mode of origin and development are chlamydospores. When the latter germinate a slender “promycelium” is put out; in Ustilago and its allies this is transversely septate, and bears lateral conidia (sporidia); in Tilletia and its allies non-septate, and bears a terminal tuft of conidia (sporidia) (fig. 7). Brefeld regarded the promycelium as a kind of basidium, bearing lateral or terminal conidia (comparable to basidiospores), but since the number of basidiospores is not fixed, and the basidium has not yet assumed very definite morphological characters, Brefeld termed the group Hemibasidii, and regarded them as a half-way stage in the evolution of the true Basidiomycetes from Phycomycetes, the Tilletia type leading to the true basidium (Autobasidium), the Ustilago type to the protobasidium, with lateral spores; but this view is based on very poor evidence, so that it is best to place these forms as a separate group, the Ustilaginales. The yeast-conidia, which bud off from the conidia or their resulting mycelium when sown in nutrient solutions, are developed in successive crops by budding exactly as in the yeast plant, but they cannot ferment sugar solutions. It is the rapid spread of these yeast-conidia in manure and soil waters which makes it so difficult to get rid of smuts, &c., in the fields, and they, like the ordinary conidia, readily infect the seedling wheat, oats, barley or other cereals. 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. When the flowers form, however, the mycelium sends hyphae into the young ovaries and rapidly replaces the stores of sugar and starch, &c., which would have gone to make the grain, by the soot-like mass of spores so well known as smut, &c. These spores adhere to the grain, and unless destroyed, by “steeping” or other treatment, are sown with it, and again produce sporidia and yeast-conidia which infect the seedlings. In other species the infection occurs through the style of the flower, but the fungus after reaching the ovule develops no further during that year but remains dormant in the embryo of the seed. On germination, however, the fungus behaves in the same way as one which has entered in the seedling stage. The cytology of these forms is very little known; Dangeard states that there is a fusion of two nuclei in the chlamydospore, but this requires confirmation. Apart from this observation there is no other trace of sexuality in the group.
II. Ascomycetes.—This, except in the case of a few of the simpler forms, is a very sharply marked group characterized by a special type of sporangium, the ascus. In the development of the ascus we find two nuclei at the base which fuse together to form the single nucleus of the young ascus. The single nucleus divides by three successive divisions to form eight nuclei lying free in the protoplasm of the ascus. Then by a special method, described first by Harper, a mass of protoplasm is cut out round each nucleus; thus eight uninucleate ascospores are formed by free-cell formation. The protoplasm remaining over is termed epiplasm and often contains glycogen (fig. 8). In some cases nuclear division is carried further before spore-formation occurs, and the number of spores is then 16, 32 and 64, &c.; in a few cases the number of spores is less than eight by abortion of some of the eight nuclei. The ascus is thus one of the most sharply characterized structures among the fungi.
| From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
| Fig. 8.—Development of the Ascus. |
| A-C, Pyronema confluens. (After Harper.) D, Young ascus of Boudiera with eight spores. (After Claussen.) |