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.