308 TRIPE DE ROCHE. PART n. 



is at first closed, and in the genus Gyrophora the disc 

 produces a number of partial fructiferous discs on the 

 original fruit-bearing surface. As usual the cells from 

 whence the asci spring belong to the medullary stratum * 

 the border on the contrary is cortical. The thallus is 

 always foliaceous, and more or less peltate, but it becomes 

 dry and brittle when exposed to drought. Several of 

 the species have tufts of strong rootlets by which the 

 frond is fixed to rocks, and some have deep pits, with 

 corresponding swellings on the upper-surface, from 

 whence they have their popular name of Tripe de Roche. 

 These lichens, together with some species of Gyrophora, 

 afforded a miserable sustenance to Sir George Back and 

 his companions during their journey along the frozen 

 regions of Arctic America in quest of the north-west 

 passage. It is evident that plants which derive their 

 sustenance solely from the medium in which they live, 

 whether air or water, can never become a permanent 

 and wholesome food for man, though erratic lichens and 

 laver may be eaten from necessity, certainly not from 

 choice. In the Gyrophora the perithecia are convolute, 

 in the Umbilicaria they are not; but both of these 

 genera, which have their seat in Arctic Europe and 

 America, possess fertilizing particles like most of their 

 class. 



In the order Graphidei the disc is linear, simple, or 

 branched, with or without an excipulum, which is car- 

 bonaceous. Many of these lichens are crustaceous, 

 and in most of the genera the perithecia are much elon- 

 gated, pointed at both ends. When they are parallel, 

 or placed at different angles to each other, they form 

 groups like Japanese or Chinese characters, whence their 

 name. The genera are determined by the position of 

 these perithecia, which are much varied. M. Tulasne 

 has found linear male particles immersed in the crust of 

 some of these lichens. They have their principal seat in 



