THE LICHENS 367 



ceases to be filamentous, and Gleocapsa loses its blue-green colour. Trente- 

 pohlia also changes its form in the Lichen thallus but reverts to normal again 

 in free cultivation. 



There are certain vegetative structures which are peculiar to the Lichen 

 thallus. The most important are : — 



Soredia. They are small rounded bodies that consist of from one to 

 several algal cells closely surrounded by fungal hyphae. When soredia are 

 formed they can usually be recognized as a greyish powder on the upper 

 surface or at the edges of the thallus. They are dispersed by wind and form 

 a method of vegetative propagation, each soredium containing both the 

 Fungus and the Alga together. 



Isidia. They are minute stalked outgrowths from the surface of the 

 thallus, which contain both algal and fungal elements. They differ from 

 soredia in being covered with a definite cortex and they are not detachable. 

 They may be photosynthetic in function. 



Cephalodia. They are found on the upper surface of some Lichens and 

 develop as abnormal and generally dark-coloured swellings or warts. They 

 are composed of the normal Lichen hyphae but with a different algal con- 

 stituent. They may be regarded as due to the reaction of the Fungus to 

 another algal partner, and in a few cases a cephalodium may resemble 

 another known Lichen, which presumably has the same fungal constituent. 



Cyphellae. They are small roundish cavities scattered over the under 

 surface of certain Lichens. Each has a definite rim and opens into the 

 medulla of the Lichen. Probably their function is to aid aeration, as they 

 occur chiefly in species with both an upper and lower cortex. 



Reproduction 



With the exception of a few tropical species which belong to the Basidio- 

 lichenes, all known Lichens belong to the division Ascolichenes, in which 

 the fungal elements belong to the Ascomycetes, and consequently the repro- 

 ductive organs are either apothecia or perithecia. 



In the simplest or Lecideine type the apothecium consists solely of 

 fungal tissue and differs in no striking way from that found in the 

 Ascomycetes, but in the Lecanorine type (Fig. 353) the rim of the 

 apothecium has the same structure as the rest of the thallus, including 

 algal cells. 



The asci are separated by paraphyses, and the whole of the hymenial 

 surface resembles that found in the Pezizales. Each ascus is of the typical 

 Ascomycete form, and each usually has eight spores which are either simple 

 or septate. Lichen apothecia vary greatly in size and may be anything from 

 0-25 to ID mm. or more in diameter. Xot infrequently the hymenium is a 

 different colour from the thallus and is sometimes quite brilliantly coloured. 



The perithecia also, where they occur, resemble those of the Ascomycetes. 

 A complete hymenial layer may surround the inner surface of the perithecium, 



