on Cladoniae: Bilimbia microcarpa, Bacidia Beckhausii and Urceolaria scruposa, etc.
Urceolaria (Diploschistes) has a somewhat bulky crustaceous thallus which may be almost evanescent in its semi-parasitic condition, the only gonidia retained being in the margin of the apothecia. Nylander[939] found isolated apothecia growing vigorously on Cladonia squamules.
Hue[940] describes Lecanora aspidophora f. errabunda, an Antarctic lichen, as not only a wanderer but as a “shameless robber.” It is to be seen everywhere on and about other lichens, settling small glomeruli of apothecia here and there on the thallus of Umbilicariae or between the areolae of Buelliae, and always too vigorous to be ousted from its position.
Bacidia flavovirescens has been regarded by some lichenologists[941] as a parasite on Baeomyces, but recent work by Tobler[942] seems to have proved that the bright green thallus is that of the Bacidia.
c. Parasymbiosis. There are certain lichens that are obligative parasites and pass their whole existence on an alien thallus. They may possibly have degenerated from the condition of facultative parasitism as the universal history of parasitism is one of increased dependence on the host, and of growing atrophy of the parasite, but, in the case of lichens, there is always the peculiar symbiotic condition to be considered: the parasite produces its own vigorous hyphae and normal healthy fruits, it often claims only a share of the carbohydrates manufactured by the gonidia. The host lichen is not destroyed by this parasymbiosis though the tissues are very often excited to abnormal growth by the presence of the invading organism.
Lauder Lindsay[943] was one of the first to study these “microlichens” as he called them, and he published descriptions of those he had himself observed on various hosts. He failed however to discriminate between lichens and parasitic fungi. It is only by careful research in each case that the affinity to fungi or to lichens can be determined; very frequently the whole of them, as possessing no visible thallus, have been classified with fungi, but that view ignores the symbiosis that exists between the hyphae of the parasite and the gonidia of the host.
Parasitic lichens are rather rare on gelatinous thalli; but even among these, a few instances have been recorded. Winter[944] has described a species of Leptoraphis, the perithecia of which are immersed in the thallus of Physma franconicum. The host is wholly unaffected by the presence of the parasite except for a swelling where it is situated. The foreign hyphae are easily distinguishable; they wander through the thallus of the host with their free ends in the mucilage of the gonidial groups from which they evidently extract nourishment. Species of the lichen genus Obryzum are also parasitic on gelatinous lichens.
The parasitic genus Abrothallus[945] has been the subject of frequent study. There are a number of species which occur as little black discs on various thalli of the large foliose lichens. They were first of all described as parasitic fungi, later Tulasne[946] affirmed their lichenoid nature as proved by the structure, consistence and long duration of the apothecia. Lindsay[947] wrote a monograph of the genus dealing chiefly with Abrothallus Smithii (Buellia Parmeliarum) and A. oxysporus, with their varieties and forms that occur on several different hosts. In some instances the thallus is apparently quite unaffected by the presence of Abrothallus, in others, as in Cetraria glauca, there is considerable hypertrophy produced, the portion of the thallus on which the parasites are situated showing abnormal growth in the form of swellings or pustules which may be regarded as gall-formations. Crombie[948] points this out in a note on C. glauca var. ampullacea, figured first by Dillenius, which is merely a swollen condition due to the presence of Abrothallus.
The internal structure and behaviour of Abrothallus has more recently been followed in detail by Kotte[949]. He recognized a number of different species growing on various thalli of Parmelia and Cetraria, but Abrothallus Cetrariae was the only one that produced gall-formation. The mycelium of the parasite in this instance penetrates to the medulla of the host lichen as a loose weft of hyphae which are divided into more or less elongate cells. These send out side branches, which grow towards the algal cells, and by their short-celled filaments clasp them exactly in the same way as do the normal lichen hyphae. Thus in the neighbourhood of the parasite an algal cell may be surrounded by the hyphae not only of the host, but also by those of Abrothallus. The two different hyphae can generally be distinguished by their reaction to iodine: in some cases Abrothallus hyphae take the stain, in others the host hyphae. In addition to apothecia, spermogonia or pycnidia are produced, but in one of the species examined by Kotte, Abrothallus Peyritschii on Cetraria caperata, there was no spermogonial wall formed. The hyphae also penetrate the host soredia or isidia, so that on the dispersal of these vegetative bodies the perpetuation of both organisms is secured in the new growth.
Abrothallus draws its organic food from the gonidia in the same way as the host species, and possibly the parasitic hyphae obtain also water and inorganic food along with the host hyphae. They have been traced down to the rhizinae and may even reach the hypothallus, but no injury to the host has been detected. It is a case of joint symbiosis and not of parasitism. Microscopic research has therefore justified the inclusion of these and other forms among lichens.