A handbook of systematic botany

by Eugenius Warming

Among the conidio-fructifications there are, in the same way, three divisions.

1. Free conidiophores (Fig. [109]). The form of the conidiophores, the shape, and number of its spores are various. In the most highly developed Fungi, the Basidiomycetes, there are, however, special more highly developed conidiophores, the basidia, which have a definite form and spores of a definite shape and number. The conidia borne on basidia are called basidiospores.

2. Conidial-layers. (a) The SIMPLEST case of this is found when the conidiophores arise directly from the mycelium, parallel to one another, and form a flat body (e.g. Exobasidium vaccinii, Hypochnus; among the Phycomycetes, Empusa muscæ and Cystopus). (b) In a HIGHER form the conidial-layers are thick, felted threads (stroma) inserted between the mycelium and the hymenium (i.e. the region of the conidiophores). Examples are found in a section of the Pyrenomycetes (Fig. [122]). (c) The HIGHEST form has the basidial-layer, that is a conidial-layer with more highly developed conidiophores (basidia). The basidial-layer, with stroma, and the hymenium (region of the basidia), forms the basidio-fructification, which is branched in the Clavariaceæ, and hat-shaped in other Hymenomycetes (in these groups the hymenium is confined to the lower side of the pileus).

The hymenium of the conidial-layer and basidial-layer is composed entirely of conidiophores, or of conidiophores and sterile hyphæ (paraphyses) which are probably always unicellular. Paraphyses are found in Entomophthora radicans, and in certain Basidiomycetes (e.g. Corticium).

3. Conidiocarps (pycnidia). A special covering surrounds the conidia-forming elements. The inner side of this covering (peridium) bears the hymenium, i.e. those elements from which the conidia are abstricted. The conidiocarps arise either immediately from the hyphæ or from a stroma in which they are generally embedded. Conidiocarps are entirely wanting in the Phycomycetes. On the other hand they are found among the Ascomycetes and Basidiomycetes, and in the latter group the conidiocarps contain more highly differentiated conidiophores (basidia) and are known as basidiocarps. Conidiocarps with simple conidiophores, are found only among the Basidiomycetes, in the Uredinaceæ, and in Craterocolla cerasi. In the Ascomycetes (Figs. [120] d, e; [117] a, b; [123] a; [124] b) the conidiocarps are visible, as points, to the naked eye, while the basidiocarps of the Basidiomycetes (Figs. [170], [171], [173–176], [178–180]) vary from the size of a pea to that of a child’s head. The “spermogonia” of the Ascomycetes and Lichenes, are conidiocarps with small conidia (microconidia) which germinate sometimes more slowly than other conidia, and formerly were erroneously considered as male reproductive cells, and called spermatia.

The conidia of the Fungi are not primitive structures. The comparison of the sporangia and conidia among the Zygomycetes, and among the species of the genus Peronospora shows, that the conidia are aberrant formations, and that they have arisen through the degeneration of the sporangium, which, by the reduction of its spores to one, has itself become a spore.

In the genera Thamnidium and Chætocladium the gradual diminution of the sporangia, and the reduction of the number of spores can be distinctly followed. In Thamnidium the number of spores is often reduced to one, which is free in the sporangium. In Chætocladium however the sporangia are typically one-spored, the spore is always united with the sporangium, and the two become a single body, the so-called conidium, which is in reality a closed sporangium. How close is the connection between the sporangia and conidia of Thamnidium and Chætocladium, is seen from the fact that, in the conidial stage of Chætocladium the same whorl-form of branching appears as in the sporangial stages of Thamnidium chætocladioides, and also, that the conidia of Ch. fresenianum throw off the former sporangium-wall (exosporium), while Ch. jonesii germinates without shedding its exosporium. The Phycomycetes have doubtless sprung from Water-Algæ and inherit the sporangia from them. On this supposition, as the Phycomycetes assumed a terrestrial mode of life, the sporangia would become adapted to the distribution of the spores by means of the air, the sporangia would become small, contain dust-like spores, and would eventually become closed-sporangia, i.e. conidia. The conidia are a terrestrial method for the multiplication of Fungi. In the Hemiasci and the Ascomycetes the sporangia are still preserved, but in every instance they are adapted to terrestrial spore-distribution, their spores being set free on the destruction of the sporangium-wall (generally shot out) and distributed through the air. For further examples of spore-distribution see below, p. [91–93].

The reproduction of Fungi is accomplished not only by spores and conidia, but also sometimes by chlamydospores. These are fundaments[11] of sporangiophores and conidiophores, which have taken on a resting condition in the form of a spore, and are able to germinate and produce carpophores. In the formation of the chlamydospores the hyphæ accumulate reserve materials at the expense of the neighbouring cells; in the undivided hyphæ of the Phycomycetes transverse walls are formed, and finally the chlamydospores are set free by the decay of the empty cells connecting them with the mycelium. One must distinguish between oidia and true chlamydospores. The former are more simple, the latter are a somewhat more differentiated form of carpophore fundaments, which serve for propagation in the same manner as spores. In Chlamydomucor racemosus the chlamydospores grow out into the air and form differentiated carpophores. In the Autobasidiomycetes they only germinate vegetatively, and not with the formation of fructifications. From Chlamydomucor up to the Autobasidiomycetes the successive development of the fructification, which is interrupted by the formation of the chlamydospores, degenerates more and more. Among certain Ustilagineæ the chlamydospores (brand-spores) no longer germinate with the production of fructifications. In the Uredinaceæ, only one of the three chlamydospore-forms has the property of producing fructifications on germination; the other forms only germinate vegetatively, like ordinary spores, and in the same manner as the chlamydospores of the Autobasidiomycetes. In the Hemibasidii, and the Uredinaceæ, in Protomyces, the chlamydospores are the chief means of reproduction. They are found also among the Ascomycetes.

The sporangia and the conidia of the Fungi have their common origin in the sporangia of the Phycomycetes. The asci (and the Ascomycetes which are characterised by these bodies) are descended from the sporangia-forming, lower Fungi; the basidia (and the Basidiomycetes) from those which bear conidia. The sporangia of the Phycomycetes are the primitive form and the starting point for all the reproductive forms of the Fungi. The chlamydospores appear besides in all classes of Fungi as supplementary forms of reproduction, and are of no importance in determining relationships. Although the expression “fruit” must essentially be applied to true Phanerogams, yet, through usage, the term “fruit-forms,” is employed to designate the forms or means of reproduction of Fungi, and the organs of reproduction are known as organs of fructification, the sporangiophores and conidiophores as fruit-bearers (carpophores), and the sporangiocarps, conidiocarps, and basidiocarps as “fruit-bodies.”

The majority of Fungi have more than one method of reproduction, often on various hosts (Uredinaceæ). Species with one, two, or more than two methods of reproduction are spoken of as having monomorphic, dimorphic, or pleomorphic fructification. Monomorphic, e.g. the Tuberaceæ; dimorphic, Mucor, Piptocephalis, Saprolegniaceæ, Penicillium crustaceum; pleomorphic, Puccinia graminis, Capnodium salicinum (in the last species there are five methods of reproduction: yeast-like conidia, free conidiophores, conidiocarps with small and large conidia, and ascocarps).