264 BIONOMICS 



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several different hosts. In some instances the thallus is apparently quite 



unaffected by the presence oi 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' 

 points this out in a note on C. glauca var. amfullacea, figured first by 

 Dillenius, which is merely a swollen condition due to the presence of 

 Abrothallus. 



The internal structure and behaviour o^ Abrothallus has more recently 

 been followed in detail by Kottel 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 distin- 

 guished by their reaction to iodine: in some cdises 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. 



d. Parasymbiosis of Fungi. There occur on lichens, certain parasites 

 classed as fungi which at an early stage are more or less parasymbionts of 

 the host ; as growth advances they may become parasitic and cause serious 

 damage, killing the tissues on which they have settled. 



Zopf^ found several instances of such parasymbiosis in his study of 

 fungal parasites, such as Rhymbocarpus punctiformis, a minute Discomycete 

 which inhabits the thallus of Rhizocarpon geographicum. By means of 

 staining reagents he was able to trace the course of the parasitic hyphae, 



1 Crombie 1894. 2 Kotte 1910. ' Zopf 1896. 



