BRITISH LICHENS. 



169 



concealed beneath the epidermis of the bark it is said to be hypophlaodal.* 

 This is the condition in many genera, amongst which may be mentioned 

 Opcgrapha, Graphis, Arthonia, and Verrucaria. 



The Reproductive Organs of Lichens. 



Having briefly described the leading characters of the Lichen thallus, 

 our next object is to convey in as few words as possible an idea of the 

 mode of reproduction in Lichens. This is accomplished by the means of 

 spores, which are capable of germinating and throwing out from one or 

 more parts of their surface fine transparent threads called hyphce, which 

 form a thin layer by becoming interlaced together, this layer as already 

 described being the hypothallus. The spores are produced in a special 

 receptacle seated on some portion of the thallus, or buried in its substance ( 

 called the 



Apothecium.j The apothecium is capable of assuming a great number 

 of various forms, which it is necessary should be clearly understood ; but there 

 are certain essential parts which belong alike to all forms. We will select 

 as a specimen for illustration the common wall Physcia (Physcia 

 parietina,) as it is a species usually exhibiting an abundance of apothecia. 

 If a well-grown specimen of this Lichen be obtained by the student}: he 

 will observe scattered about the centre of the thallus a number of orange- 

 coloured circular shields or cups, surrounded by a pale margin, seated 

 close on the surface, or slightly raised by a broadish stem-like base ; these 

 are the apothecia (Fig. 4.) He will observe that the area enclosed by the 

 pale margin is slightly depressed, (Fig. 12,) but less so in the older 

 individuals ; this area is called the kymenium§ by some, and the thalamium\\ 

 by others, and is composed of a vast number of club-shaped cells 

 arranged in an upright position, side by side, within which are produced 

 the spores already mentioned. These club-shaped cells are the ascilT 

 or thecce,** and each one contains in the species under examination 

 eignt spores, which escape on the rapturing of the asci at maturity. 

 These asci are not always sack-shape, being sometimes narrowly 

 cylindrical, at others nearly spherical, with numerous intermediate 

 variations. 



The spores vary in number in each ascus, according to species, from 

 a single one to a great number, but the most common number is eight. 

 They also vary very much in size, form, mode of internal division, and 

 colour. It is by the aid of these characters that species can be satis- 



* From i'lrb under, and 4>\oi6s the bark of a tree. 



t From a7rd t upon, and Ov K V, a sac, the part bearing the a?ci. 



t To meet the case of such a; have no means of obtaining specimens of the 

 commoner species of the British Lichens, the author of this paper has determined to 

 prepare small collections, correctly named, to be obtained post free on application 

 to his address, Canonbury, Shrewsbury, at the following rates :— Elementary 

 Collections of British Lichens, Series I., containing twenty-five specimens, 5e. 6d.'; 

 Series II, containing fifty specimens, 10s. ; Series III, containing one hundred 

 specimens, 18s. 



§ From <Wi a membrane. 

 || From OdXafios, a receptacle. 

 U From d<r/cds, a leather bottle. 

 ** From OrjKT], a sack. 



