INTRODUCTION 



LICHENS are, with few exceptions, perennial aerial plants of somewhat 

 lowly organization. In the form of spreading encrustations, horizontal leafy 

 expansions, of upright strap-shaped fronds or of pendulous filaments, they 

 take possession of the tree-trunks, palings, walls, rocks or even soil that 

 afford them a suitable and stable foot-hold. The vegetative body, or thallus, 

 which may be extremely long-lived, is of varying colour, white, yellow, 

 brown, grey or black. The great majority of lichens are Ascolichens and 

 reproduction is by ascospores produced in open or closed fruits (apothecia 

 or perithecia) which often differ in colour from the thallus. There are a few 

 Hymenolichens which form basidiospores. Vegetative reproduction by 

 soredia is frequent. 



Lichens abound everywhere, from the sea-shore to the tops of high 

 mountains, where indeed the covering of perpetual snow is the only barrier 

 to their advance ; but owing to their slow growth and long duration, they 

 are more seriously affected than are the higher plants by chemical or other 

 atmospheric impurities and they are killed out by the smoke of large towns: 

 only a few species are able to persist in somewhat depauperate form in or 

 near the great centres of population or of industry. 



The distinguishing feature of lichens is their composite nature : they 

 consist of two distinct and dissimilar organisms, a fungus and an alga, which, 

 in the lichen thallus, are associated in some kind of symbiotic union, each 

 symbiont contributing in varying degree to the common support : it is 

 a more or less unique and not unsuccessful venture in plant-life. The 

 algae Chlorophyceae or Myxophyceae that become lichen symbionts or 

 "gonidia" are of simple structure, and, in a free condition, are generally to 

 be found in or near localities that are also the customary habitats of lichens. 

 The fungus is the predominant partner in the alliance as it forms the fruiting 

 bodies. It belongs to the Ascomycetes 1 , except in a few tropical lichens 

 (Hymenolichens), in which the fungus is a Basidiomycete. These two 

 types of plants (algae and fungi) belonging severally to many different 

 genera and species have developed in their associated life this new lichen 

 organism, different from themselves as well as from all other plants, not 

 only morphologically but physiologically. Thus there has arisen a distinct 

 class, with families, genera and species, which through all their varying forms 

 retain the characteristics peculiar to lichens. 



1 E. Acton (1909) has described a primitive lichen Botrydina viilgaris, in which there is no 

 fruiting stage, and in which the fungus seems to show affinity with a Hyphomycete. 



