II. SECONDARY SPORES
A. Reproduction by Oidia
In certain conditions of nutrition, fungal hyphae break up into separate cells, each of which functions as a reproductive conidium or oidium, which on germination forms new hyphae. Neubner[690] has demonstrated a similar process in the hyphae of the Caliciaceae and compares it with the oidial formation described by Brefeld[691] in the Basidiomycetes.
The thallus of this family of lichens is granular or furfuraceous; it never goes beyond the Lepra stage of development[692]. In some species it is scanty, in others it is abundant and spreads over large areas of the trunks of old trees. It is only when growth is especially luxuriant that oidia are formed. Neubner was able to recognize the oidial condition by the more opaque appearance of the granules, and under the microscope he observed the hyphae surrounding the gonidia gradually fall away and break up into minute cylindrical cells somewhat like spermatia in size and form. There was no question of abnormal or unhealthy conditions, as the oidia were formed in a freely fruiting thallus.
The gonidia associated with the oidial hyphae also showed unusual vitality and active division took place as they were set free by the breaking up of the encircling hyphae. The germination of the oidia provides an abundance of hyphal filaments for the rapidly increasing algal cells, and there follows a wide-spread development of the lichen thallus.
Oidial formation has not been observed in any other family of lichens.
B. Reproduction by Conidia
a. Instances of Conidial Formation. It is remarkable that the type of asexual reproduction so abundantly represented in fungi by the large and varied group of the Hyphomycetes is practically absent in lichens. An exception is to be found in a minute gelatinous lichen that grows on soil. It was discovered by Bornet[693] and called by him Arnoldia (Physma) minutula. From the thallus rise up simple or sparingly branched colourless conidiophores which bear at the tips globose brown conidia ([Fig. 108]). Bornet[694] obtained these conidia by keeping very thin sections of the thallus in a drop of water[693].
Fig. 108. Conidia developed from thallus of Arnoldia minutula Born. × 950 (after Bornet).
Yet another instance of conidial growth is given by Steiner[695]. He had observed that the apothecia on plants of Caloplaca aurantia var. callopisma Stein. differed from those of normal appearance in the warted unevenness of the disc and also in being more swollen and convex, the thalline margin being almost obliterated. He found, on microscopical examination, that the hymenium was occupied by paraphyses and by occasional asci, the latter seldom containing spores, and being usually more or less collapsed. The component parts of the apothecium were entirely normal and healthy, but the paraphyses and the few asci were crushed aside by the intrusion of numerous slender unbranched septate conidiophores. Several of these might spring from one base and the hypha from which they originated could be traced some distance into the ascogenous layer, though a connection with that cell-system could not be demonstrated. While still embedded in the hymenium, an ellipsoid or obovate swelling began to form at the apex of the conidiophore; it became separated from the stalk by a septum and later divided into a two-celled conidium. The conidiophore increased in length by intercalary growth and finally emerged above the disc; the mature conidium was pyriform and measured 15-20 µ × 9-11 µ.
Steiner regarded these conidia as entirely abnormal; pycnidia with stylospores are unknown in the genus and they were not, he alleges, the product of any parasitic growth.
b. Comparison With Hyphomycetes. The conidial form of fructification in fungi, known as a Hyphomycete, is generally a stage in the life-cycle of some Ascomycete; it represents the rapid summer form of asexual reproduction. The ascospore of the resting fruit-form in many species germinates on any suitable matrix and may at once produce conidiophores and conidia, which in turn germinate, and either continue the conidial generation or proceed to the formation of the perfect fruiting form with asci and ascospores.
Such a form of transient reproduction is almost impossible in lichens, as the hypha produced by the germinating lichen ascospore has little vitality without the algal symbiont. In natural conditions development practically ceases in the absence of symbiosis. When union between the symbionts takes place, and growth becomes active, thallus construction at once commences. But in certain conditions of shade and moisture, only the rudiments of a lichen thallus are formed, known as a leprose or sorediose condition. Soredia also arise in the normal life of many lichens. As the individual granules or soredia may each give rise to a complete lichen plant, they may well be considered as replacing the lost conidial fructification.
C. Campylidium and Orthidium
Müller[696] has described under the name Campylidium a supposed new type of asexual fructification which he found on the thallus of tropical species of Gyalecta, Lopadium, etc., and which he considered analogous to pycnidia and spermogonia. Wainio[697] has however recognized the cup-like structure as a fungus, Cyphella aeruginascens Karst., which grows on the bark of trees and occasionally is parasitic on the crustaceous thallus of lichens. Wainio has also identified the plant, Lecidea irregularis, first described by Fée[698], as also synonymous with the fungus.
Another name Orthidium was proposed by Müller[699] for a type of fructification he found in Brazil which he contrasts or associates with Campylidium. It has an open marginate disc with sporophores bearing acrogenous spores. He found it growing in connection with a thin lichen thallus on leaves and considered it to be a form of lichen reproduction. Possibly Orthidium is also a Cyphella.