30 LICHENS. 



indented at the edges, or pendent in long, somewhat rigid, and 

 beard-like filaments. This membranous substance of various 

 form is called the thalliis^ and represents a very well-known and 

 common, but by no means universal, thalline form. Many thalli 

 are scaly, or scurfy, or powdery, or gelatinous ; and many again 

 exist only during the early stages of the Lichen's life, disappear- 

 ing completely eventually. Still, all Lichens, with the exception 

 of a few parasitic species, have a thallus of one kind or another, 

 at least for a time ; and all thalli have much the same structure. 

 Three distinct layers are almost always present, called respec- 

 tively the cortical, gonidial, and medullary layers. The cortical 

 layer forms the upper surface — the bark, as it were — of the 

 thallus, and is composed of minute cells closely compacted 

 together. Beneath this is the gonidial layer, consisting of a 

 series of cells, filled with a green colouring matter, which seem 

 to lie close together, but without any actual union, and which are 

 called gonidia. These gonidia, which have the power of 

 reproduction by bisection, or splitting into parts (like many 

 Algae), are at the present day the subject of very warm discus- 

 sion amongst cryptogamic botanists, and upon them is based a 

 controversy which affects so important a point as the right of 

 Lichens to rank as a distinct order of plants. No adequate 

 distinction has hitherto been recognised between the Lichens and 

 the Fungi, except the presence in the former of those spherical 

 green bodies ; and a theory has of late years been broached, and 

 obtained some adherents, that these gonidia are in reality uni- 

 cellular Algae, upon which various Fungi, constituting the residue 

 of the so-called Lichens, are parasitic. According to this 

 dual theory of Lichens, as it has been termed, these plants are 

 merely a composition of Algae and Fungi ; and if it has met with 

 some clever exponents and defenders, it still lies open to very 

 serious objections, and is considered untenable by many of our 

 leading cryptogamists. 



There still remains the medullary layer — a mass of colour- 

 less, interwoven filaments, from which, in many of the foliaceous 

 species, root-like organs spring, serving to attach the plant firmly 

 to the substance on which it grows. A vertical section through 

 the thallus of Peltigei^a canuia — a large, membranous Lichen of 

 an olive-green colour, turned up with brown, to describe it 

 roughly — shows these layers readily under the microscope. Where 

 the air is unfavourable to the growth of Lichens — and these 

 plants never flourish where the atmosphere is impure — masses or 

 communities of gonidia will increase and form pseudo-thalli ; 

 but the reproduction of the perfect Lichen is effected by special 

 organs — spermatia and sporidia, enclosed in special receptacles — 

 spermogotiia and apothccia. The spermogonia are usually very 



