More success attended Tobler’s[500] attempt to cultivate the soredia of Cladonia sp. He sowed them on soil kept suitably moist in a pot and after about nine months he obtained fully formed squamules, at first only an isolated one or two, but later a plentiful crop all over the surface of the soil. Tobler also adds that soredia taken from a Cladonia, that had been kept for about half a year in a dry room, grew when sown on a damp substratum. The algae however had suffered more or less from the prolonged desiccation, and some of them failed to develop.
A suggestion has been made by Bitter[501] that a hybrid plant might result from the intermingling of soredia from the thallus of allied lichens. He proposed the theory to explain the great similarity between plants of Parmelia physodes and P. tubulosa growing in close proximity. There is no proof that such mingling of the fungal elements ever takes place.
D. Evolution of Soredia
Soredia have been compared to the gemmae of the Bryophytes and also to the slips and cuttings of the higher plants. There is a certain analogy between all forms of vegetative reproduction, but soredia are peculiar in that they include two dissimilar organisms. In the lichen kingdom there has been evolved this new form of propagation in order to secure the continuance of the composite life, and, in a number of species, it has almost entirely superseded the somewhat uncertain method of spore germination inherited from the fungal ancestor, but which leaves more or less to chance the encounter with the algal symbiont.
From a phylogenetic point of view we should regard the sorediate lichens as the more highly evolved, and those which have no soredia as phylogenetically young, though, as Lindau[502] has pointed out, soredia are all comparatively recent. They probably did not appear until lichens had reached a more or less advanced stage of development, and, considering the polyphyletic origin of lichens, they must have arisen at more than one point, and probably at first in circumstances where the formation of apothecia was hindered by prolonged conditions of shade and moisture.
That soredia are ontogenetic in character, and not, as Nilson[503] has asserted, accidental products of excessively moist conditions is further proved by the non-sorediate character of those species of crustaceous lichens belonging to Lecanora, Verrucaria, etc. that are frequently immersed in water. Bitter[504] found that the soredia occurring on Peltigera spuria were not formed on the lobes which were more constantly moist, nor at the edges where the cortex was thinnest: they always emerged on young parts of the thallus a short way back from the edge.
Bitter[504] points out that in extremely unfavourable circumstances—in the polluted atmosphere near towns, or in persistent shade—lichens, that would otherwise form a normal thallus, remain in a backward sorediose state. He considers, however, that many of these formless crusts are autonomous growths with specific morphological and chemical peculiarities. They hold these outposts of lichen vegetation and are not found growing in any other localities. The proof would be to transport them to more favourable conditions, and watch development.
4. ISIDIA
A. Form and Structure of Isidia
Many lichens are rough and scabrous on the surface, with minute simple or divided coral-like outgrowths of the same texture as the underlying thallus, though sometimes they are darker in colour as in Evernia furfuracea. They always contain gonidia and are covered by a cortex continuous with that of the thallus.