470 SYSTEMATIC BOTANY. 



then spring from the primitive coil of the ascogonium and increase in 

 number with the formation of the hymenium, on which ultimately the 

 asci arise from the ascogenous threads. The production of spores by the 

 asci terminates the generation. 



As to the relations between the parasite and host, the following instance 

 will prove illustrative. The Algae embedded in the host received the name 

 of gonidia before their nature was truly understood, and it was generally 

 supposed that they were special organs. A smaller form of gonidia, called 

 the hymenial gonidia, was found to occur also in the empty spaces of the 

 apothecia of many Lichens. They are the offspring of the true gonidia by 

 division, and are earned up in the hymenium by the growth of the hyphse. 

 They are cast out of the apothecia along with the spores, and the spores 

 on germinating envelope with their germ-tubes the hymenial gonidia, 

 which increase in size and become the thallus-gonidia of the new lichen. 

 This has been observed in Dermatocarpon Schaereri and Polyblastia rugu- 

 losa by Dr. Stahl. Beside the Dermatocarpon there grows a species of 

 Thclidium the gonidia of which are the same species of alga (of the genus 

 Pkurococcus} as those of the genus Dermatocarpon. If the spores only of 

 the TJielidium be brought together with no other organisms than the 

 hymenial gonidia of the Dermatocarpon, the thallus of the Thelidium 

 with the characteristic fructification may be obtained on a suitable sub- 

 stratum, thus proving that the same species of gonidia can nourish two 

 Fungi of even different genera. It must not be assumed that the above 

 history of the gonidia is true of all Lichens under all circumstances, since 

 no doubt the great majority of Lichens find their hosts in a more acci- 

 dental fashion. The gonidia reproduce themselves exactly like the free 

 individual Algse of the same species or genus. 



Distribution. Lichens grow mostly in exposed situations, such as on 

 rocks, walls, trees, &c. in all parts of "the globe. They form a very large 

 proportion of the entire vegetation in the higher regions of mountains and 

 in polar latitudes. The thallus has usually a dry, dead-looking aspect 

 (though sometimes soft and pulpy), and is of a foliaceous or scaly and 

 crustaceous form. It varies much in size. 



Qualities and Uses. Many Lichens are very nutritious ; a number of 

 them yield valuable dyes ; some are medicinal, others aromatic. Among 

 the more important nutritious kinds are : Cladonia rangiferina, " Reindeer 

 Moss ; " Cetraria islandica, Iceland Moss, and C. nivalis ; Umbilicaria 

 (various species), constituting " Tripe-de-roche" of the North- American 

 hunters ; Lecanora esculenta (Tartary) and L. affinis, Sticta pulmonaria, 

 c. From Lecanora tartarea, the purple dye called Cudbear is obtained ; 

 Parmelia parietina, common on walls and roofs, gives a yellow colour ; 

 Roccella tinctoria (Mediterranean and Cape-Verd Islands, &c.), R. fu- 

 ciformis (Madeira, Angola, Madagascar, S. America), and R. hypomecha 

 are Orchil-weeds, from which the dyeing material Orchil or Orchel is ob- 

 tained litmus being obtained from* thes^ and other species of Roccella. 

 Some species contain a considerable quantity of oxalate of lime in the 

 form of crystals. 



