CHAPTER IX 



THE LICHENS 



Mention has already been made of the association of Bacteria with parts 

 of flowering plants to their mutual advantage. In the Lichens a similar type 

 of association invariably occurs, only in this instance the association is between 

 an Alga and a Fungus. The Lichens afl^ord the best example of symbiosis 

 met with in the plant kingdom. In this symbiosis the Fungus provides the 

 body of the organism, while the Alga, by virtue of the chlorophyll present in 

 its cells, is enabled to manufacture food both for itself and also for the cells 

 of the Fungus body in which it is immersed. 



The Lichen thallus, which is essentially fungal in nature, varies 

 considerably in shape, and in some species is highly developed and complex 

 both in form and structure. The reproductive organs too are fungal in 

 character, and bv means of them the Lichens can be related to certain well- 

 known fungal families. The algal constituent, on the other hand, is relatively 

 simple. In most it is unicellular, though in a few Lichens it is filamentous. 



With very few^ exceptions the Fungus is a member of the Ascomycetes, 

 and the reproductive bodies are either apothecia or, in certain genera, 

 perithecia. In one or two tropical Lichens the Fungus is a member of the 

 Basidiomycetes. The Alga belongs either to the Chlorophyceae or to the 

 Cyanophyceae, and in the majority of Lichens the algal constituent is a 

 member of either the Chlorococcales or Chaetophorales. 



Classification depends chiefly on the form of the fungal element, and the 

 whole group is separated into two main divisions : — 



1. Ascolichenes, in which the Fungus is an Ascomycete. 



(i) Gymnocarpeae, with apothecia. 

 (2) Pyrenocarpeae, with perithecia. 



2. Basidiolichenes or Hymenolichenes, in which the Fungus is a 

 Basidiomycete. 



The dual nature of Lichens was not recognized until comparatively 

 recently. As late as 1867 it was thought that they w^ere independent 

 organisms, the Algae being regarded as gonidia or asexual reproductive 

 cells. In that year Schwcndener published a paper in which he suggested 

 that the green cells in Lichens might in reality be Algae. He thought that the 

 Fungus acted simply as a parasite and that the Alga was eventually killed 

 by it, though at first it was stimulated to more vigorous grow-th. This is 

 undoubtedly too simple a view, but the idea of symbiosis w^as not reached 

 until 1873. Schwendener's " Dual Flypothesis," as it is called, excited great 

 criticism, but in 1871 Rees attempted to put the matter to proof by trying 



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