170 BRITISH LICHENS. 



factorily determined, as each particular species exhibits its own 

 characteristic spore with wonderful constancy. The spores are believed 

 to be the female organs of the plant, and to acquire their germinating 

 power from a process of impregnation by bodies we shall proceed 

 presently to describe ; but in what stage of their existence this process 

 takes place has not yet been satisfactorily shown. 



Returning to the hymenium, the student will find other upright 

 bodies intermixed with the asci, surrounding them on all sides, exceeding 

 them slightly in length, but much slenderer, and thicker at the summits 

 than below; these are the Paraphyses* (Fig. 13a.) The precise function 

 of these bodies has not been ascertained, but there can be no doubt they 

 serve as a protection to those very important bodies the asci, whatever 

 other functions they may perform. They occur in some species as very 

 slender threads, either simple or branched, and are also often so united 

 together as to be inseparable by pressure ; they are then said to be 

 indistinct. 



This layer, called the hymenium or thalamium, composed of asci 

 and paraphyses, is permeated by a substance called the hymenial 

 gelatine, which holds the whole together in a compact mass. In pro- 

 ceeding downwards in our examination of the apothecium, we come to 

 a layer of small cells, which forms the bed from which springs the 

 hymenium. (Fig. 13c.) This layer is called the hypothecium,f because 

 it underlies the asci or thecas. The colour of this forms an important 

 character in the large genus Lecidea, being either colourless or dark 

 coloured. We have finally to point out the excipulum, or receptacle 

 within which the previously described parts of the apothecium are 

 contained. In the species under examination this is formed by a 

 continuation of the substance of the thallus, which has taken the shape 

 of a miniature cup, in the bottom of which the hymenium and the 

 hypothecium are seated. (Figs. 11, 12.) If a perpendicular section be 

 made through the middle of the apothecium, carrying it down through 

 the thallus, there will be no difficulty in seeing, with a pocket lens, the 

 arrangement of all the parts in the order we have here described them. 

 It is only in certain genera that the excipulum is formed of the same 

 tissue as the thallus. It is often formed of its own special tissue, in 

 which case it is called a proper excipulum ; and the margin it forms 

 when looked at from above is called a proper margin, whereas in the 

 former case it is called a thalline excipulum and a thalline margin. 



There is yet one more term used in reference to the apothecium, 

 which it is necessary to explain — the epithecium.\ This term is used to 

 denote the upper surface of the hymenium, the part that is presented to 

 the light. 



The various parts of the apothecium we have described are essen- 

 tially the same in all, though assuming a multitude of different forms ; 



* From Trapa, about, and <p6u, to grow, 

 t From vwb, under, and Gv K Vt a sack. 

 t From M) upon, and O'faVt a sack. 



