BRITISH LICHENS. 25 



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 

 ax-e the apothecia (Fig. 4.) He will observe that tbe area enclosed by the 

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

 individuals ; this area is called the hymenium* 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 asci 

 or thcc(c,% and each one contains in the species under examination 

 eight spores, which escape on the rupturing of the asci at maturity 

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

 cylindi'ical, 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- 

 factorily determined, as each particular species exhibits its own 

 characteristic spore with wonderful constancy. The spores are beheved 

 to be the female organs of the plant, and to acquire their gex-minating 

 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 Parapliyses.W (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 hymeuial 

 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 ceUs, which forms the bed fi-om which springs the 

 hymenium. (Fig. 13c.) This layer is called the hypothecium,1f because 

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

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



* From ^M^j a membrane. 



+ From OdXafios, a receptacle. 



; From dc/cos, a leather bottle 



§ From 6v'<''}t a sack. 



II From irapa, about, and <pvo}, to grow. 



% From ^TTo, under, and 6-qKri, a sack. 



