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.