646 LICHENES. 



2. Gasterothalamese {yaaT'fip^ a belly) : shields either closed always, or opening 



hy bursting through the cortical layer of the thalius, the nucleus containing 

 the deliquescing or shrivelled sporangia. 



3. Idiothalamece (iSios, peculiar) : shields closed at first, opening afterwards, con- 



taining free spores in a nucleus composed of the gelatinous remains of the 

 paraphyses and sporangia. 



4. Coniothalamese (kSpis, powder), pulverulent lichens ; shields open, without a 



nucleus, cavity filled with free spores. 



Lichens are found in all quarters of the globe, adhering to stones, 

 rocks, trees, etc. They derive much of their nourishment from the 

 atmosphere. They have the power of acting on hard rocks, so as to 

 disintegrate them ; and many of them contain much inorganic matter 

 in their composition. They all grow in the air ; none are found sub- 

 mersed. Genera, 60 ; species, 2400. Examples — Urceojaria, UmhOi- 

 caria, Lecidea, Cladonia, Parmelia, Cetraria, Eoccella, Evernia. 



The Thalius of Lichens is composed of a filamentous tissue called 

 hypha {'J'pfi, a weaving), the filaments of which are usually colourless, 

 and of green, yellow, blue, or brown cellules, called gonidia {yovog, off- 

 spring), which vary as regards their character and situation. They 

 contain either chlorophyll or a colouring matter called phycochrome 

 (puxoe, seaweed, -x^^u/jja, colour), which distinguishes an entire group 

 of the lower algae. The hypha forms the principal part of the thalius. 

 It may be in a filamentous form, or it may be developed as fronds of 

 considerable extent. Lichens are connected with Algae on one hand 

 by means of some of the CoUemaceae, and with Fungi on the other hy 

 the inferior genera of Pjrrenocarpei. Of late a singular hypothesis has 

 been brought forward by Schwendener to the eflfect that Lichens are 

 not autonomous plants, but are composed of a true algal and a para- 

 sitic fungus. Each lichen is supposed to be an algal-type, which has 

 become the host of a parasitic fungus growth ; the Lichfen-gonidia 

 being algae, and the Lichen-thallus (the hypha), a parasitic fungus. 

 This theory is illustrated by Nostoc, an independent algal, which 

 may either continue so, or it may become the liost of a parasitic 

 fungus, and by it be converted into a Collema, or what is usually 

 called Lichen.* This view is not adopted by our best fungologists. 

 (For a detailed account of the reproductive process in Licheiis, see 

 p. 268.) 



Lichens furnish articles of food and important dyes. Cetraria 

 ■ islandica, commonly called Iceland Moss, contains a nutritious matter 

 called Lichenin, or Lichen-starch. There exists in it a bitter principle 

 also, to which the name Cetrarin has been given. The plant is used 

 as a demulcent and tonic, in the form of decoction or jelly. This 

 Lichen occurs in northern regions, as Greenland, Iceland, Spitzbergen, 

 and Scandinavia, on the mountains of Britain, and other parts of 



* See a paper by Mr. Crombie, in Popular Science RevieWj July 1874. See also Ch-eviUea, 

 1873, and Ann. des Sc. Nat. 5 ser. xvii. 



