24 BEITISH LICHENS. 



cortical, gonidial, and medullary layers in precisely the same order as above 

 but instead of rhizimewe should have seen a very thin layer of dead cellular 

 black matter adhering closely to the surface of the atone, which is called 

 the ]njpothaUus.* This is formed by the threads first thrown out by the 

 germinating spores, and is the earliest stage of a Lichen thallus. On 

 quartz rock this hypothallus of Lecidea Qeographica is often to be seen in 

 black radiating lines, while as yet no other part of the thallus has been 

 formed. 



In some of the gelatinous Lichens, as for example in Collema, the 

 cortical layer is absent, while in an allied genus, Leptogium, it is reduced 

 to the thinnest possible layer ; the gonidial layer and the medullary are 

 also mixed up together in one common stratum. (Plate V., Fig. 10.) 



Wliile a well developed Lichen thallus will have the characters above 

 described, it must not be forgotten that in many species it is reduced to 

 such insignificance that it consists of little more than a few filaments, 

 accompanied by a few gonidia lying beneath the epidermis of the bark of 

 a tree, or running through the disintegrated surface of a rock. In species 

 with such an imperfectly developed thallus specimens will occur in which 

 it cannot be detected, it is then said to be evanescent ; but if present and 

 concealed beneath the epidermis of the bark it is said to be hypoplilaodal.] 

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

 Opegrapha, Graphis, Arthonia, and Verrucaria. 



The Eepkoductive 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 gei'minating and throwing out from one or 

 more parts of their surface fine transparent threads called hiiplur, 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.\ The apothecium is capable of assummg a great number 

 of various f orms,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- 



* From vt6 under, and da.Wh% a frond, i.e., that which underlies the thallus. 



t From vTtb under, and ^\oi.bs the bark of a tree. 



I From dvb, upon, and O'^kt], a sac, the part bearing the asci. 



§ 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, Canoubnry, Shrewsbury, at the following rates :— Elementary 

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

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

 specimens, 18s. 



