190 



REPRODUCTION 



The thallus of this family of lichens is granular or furfuraceous ; it never 

 goes beyond the Lepra stage of development 1 . In some species it is scanty, 

 in others it is abundant and spreads over large areas of the trunks of old 

 trees. It is only when growth is especially luxuriant that oidia are formed. 

 Neubner was able to recognize the oidial condition by the more opaque 

 appearance of the granules, and under the microscope he observed the 

 hyphae surrounding the gonidia gradually fall away and break up into 

 minute cylindrical cells somewhat like sperm.atia in size and form. There 

 was no question of abnormal or unhealthy conditions, as the oidia were 

 formed in a freely fruiting thallus. 



The gonidia associated with the oidial hyphae also showed unusual 

 vitality and active division took place as they were set free by the breaking 

 up of the encircling hyphae. The germination of the oidia provides an 

 abundance of hyphal filaments for the rapidly increasing algal cells, and 

 there follows a widespread development of the lichen thallus. 



Oidial formation has not been observed in any other family of lichens. 



B. REPRODUCTION BY CONIDIA 



a. INSTANCES OF CONIDIAL FORMATION. It is remarkable that the 

 type of asexual reproduction so abundantly represented in fungi by the large 



and varied group of the Hyphomycetes is prac- 

 tically absent in lichens. An exception is to be 

 found in a minute gelatinous lichen that grows on 

 soil. It was discovered by Hornet 2 and called by 

 him Arnoldia (Physma) minutula. From the thallus 

 rise up simple or sparingly branched colourless 

 conidiophores which bear at the tips globose brown 

 conidia(Fig. 108). Bornet 3 obtained these conidia 

 by keeping very thin sections of the thallus in a 

 drop of water 2 . 



Yet another instance of conidial growth is given 

 by Steiner 4 . He had observed that the apothecia 

 on plants of Caloplaca aurantia var. callopisma 

 Stein, differed from those of normal appearance 

 in the warted unevenness of the disc and also in 

 being more swollen and convex, the thalline margin 

 being almost obliterated. He found, on micro- 

 scopical examination, that the hymenium was 

 occupied by paraphyses and by occasional asci, 

 the latter seldom containing spores, and being 



2 Bornet 1873. 



* Bornet's observations have not been repeated, and it is possible that he may have been dealing 

 with a parasitic hyphomycetous fungus. 4 Steiner 1901. 



Fig. 1 08. Conidia developed 

 from thallus of Arnoldia mi- 

 nutula Born, x 950 (after 

 Bornet). 



1 See p. 143. 



