268 BIONOMICS 



and feeds on the living hyphae. It is a minute species, but when abundant 

 the Plasmodia can just be detected with the naked eye as rosy specks 

 scattered over the surface of the lichen. Later the grey sporangia are 

 produced on the same areas. 



F. Diseases of Lichens 



a. Caused by Parasitism. Zopf ^ has stated that of all plants, lichens 

 are the most subject to disease, reckoning as diseases all the instances of 

 parasitism by fungi or by other lichens. There are however only rare 

 instances in which total destruction or indeed any permanent harm to the 

 host is the result of such parasitism. At worst the trouble is localized and 

 does not affect the organism as a whole. Some of these cases have been 

 already noted under antagonistic symbiosis or parasymbiosis. Several 

 instances have however been recorded where real injury has been caused 

 by the penetration of some undetermined fungus mycelium. ZukaP records 

 two such observed by him in Parmelia encausta and Physcia villosa : the 

 thallus of the former was dwarfed and deformed by the presence of the alien 

 mycelium, the latter was excited to abnormal proliferation. 



b. Caused by crowding. Lichens suffer frequently from being over- 

 grown by other lichens ; they may also be crowded out by other plants. 

 My attention was called by Mr P. Thompson to a burnt plot of ground in 

 Epping Forest, which, after the fire, had been colonized by Peltigera spuria. 

 In the course of a few years, other vegetation had followed, depriving the 

 lichen of space and light and gradually driving it out. When last examined 

 only a few miserable specimens remained, and these were reduced in vitality 

 by an attack of the lichen parasite Illosporium carneum. 



c. Caused by adverse conditions. Zukal considers as pathological, 

 at least in origin, the cracking of the thallus so frequent in crustaceous 

 lichens as well as in the more highly developed forms. As the cracks are 

 beneficial in the aeration of the plant, they can hardly be regarded as 

 symptoms of a diseased condition. The more evident ringed breaks in the 

 cortex of Usneae, due probably to wind action, have more reason to be so 

 regarded ; they are most pronounced in Usnea articulata, where the portions 

 bounded by the rings are contracted and swollen, and a hollow space is 

 foi-med between the cortex and the central axis. The swellings that are 

 produced on lichen thalli, such as those of Umbilicaria and some species of 

 Gyrophora, due to intercalary growth are normal to the plant, though occasion- 

 ally the swollen weaker portions may become ruptured and the cortex be 

 thrown off. As pathological also must be regarded the loss of cortex some- 

 times occasioned by excessive soredial formation at the margins of the lobes: 



1 Zopf 1897. 2 Zukal 1896, p. as8- 



