GROUPS OF PLANTS. 



39 



duces, like many other Lichens, disk-hke or cup-shaped l)0(Ucs at 

 various places on the surface of the thallus, which are known as 

 APOTHECiA and which may be regarded as exposed or open asco- 

 carps. The inner surface of the apothecia is lined with a number 

 of asci as well as sterile cells, the former giving rise to ascospores. 



Fig 26. Iceland Moss (Ceirariaislandica). A-F, various forms of thalli showing apoth- 

 ecia (a)- I. cross-section of an apothecium showing the hymenium (h), the hypothecmm 

 (p), the algal layer (e), the medullary layer (m), and lower or ventral surface (1); K, an 

 ascus with eight ascospores and two paraphyses from the hymenium (h of I). 



Economic Uses of Lichens. A number of the Lichens are 

 used in medicine, as several species of Cetraria, Pertusana com- 

 munis. Physcia parietina, Sticta pnlmomcea, Evernia furfuracea. 



