1903-1904-] -^ Short Talk on Lichens. 91 



capable. Some lichens have no proper thallus, and some grow 

 under the bark of trees, as Arthonia. Excrescences on the 

 thallus are soredia or protruding gonidia capable of propagat- 

 ing some species, like the gemmae of mosses and hepaticse. 

 The stratified cellular layers of the lichen proper are the 

 cortical layer, the gonidial layer, and the medullary layer. 

 The medullary layer in the highly branched species, as 

 Usnea and Cladonia, forms a tough cord in the centre, 

 which is at length exposed by the cracking of the cortical 

 layer. 



Lichens are said to have many modes of reproduction, some 

 authorities giving as many as six. The normal method of 

 fructification is by spores developed in a special organ called 

 the apothecium, which is always found upon the surface or 

 attached to the edge of the thallus. In Cladoniae the apothecia 

 surmount the tops of podetia, which are cylindrical and vertical 

 prolongations of the thallus, and which may be regarded as 

 modifications of the simple thalline foliole or squamule. In 

 Cladonife the podetia are crowned with a cup-like cavity, on 

 the toothed margins of which grow the apothecia. Sometimes 

 the cup is replaced by globose fruit either singly or con- 

 glomerate. The Cladoniae, from their having a vertical as 

 well as a horizontal, a secondary as well as a primary thallus, 

 are ranked among the highest typical forms of lichens. 

 Popularly, but erroneously, they are named " cup-mosses." 

 This family of lichens is foliaceous or fruticulose, having 

 fistulose podetia bearing the apothecia in scyphi or cups, or 

 in globose masses. The thallus is generally foliaceous or 

 squamulose at the base, and sometimes granulose or crusta- 

 ceous, &c. Cladonise are divided into three groups, which is 

 the first step in their determination. These groups are 

 Pycnothelia, of two species in Britain ; Cladonia, with about 

 thirty-four species ; and Cladina, with four or five species. 

 The second step in their determination is to carefullly notice 

 the colour of the apothecia, and whether the apothecia are 

 situated on scyphi or not. The two divisions here are 

 Pheeocarpae, with apothecia pale or flesh-coloured turning 

 fuscous when old, and Erythrocarpae, with apothecia red like 

 red sealing-wax. You will see from this that the presence of 

 apothecia on Cladoniae is indispensable to their correct deter- 



