GENERAL CHARACTERS OF BRITISH LICHENS. 41 



play ill the multiplication and modification of the Lichen- 

 thallus, they demand more than a passing allusion. They 

 may be regarded as intermediate in function between the 

 vegetative and reproductive cell, assuming the offices and 

 partaking of the characters of both ; but as they are peculiar 

 to, and characteristic of, the thallus of Lichens, — serving to 

 distinguish them from cognate families, — and as their hyper- 

 trophic or abnormal condition is one chief cause of irre- 

 gularity in the external characters of the thallus, we prefer 

 considering them here. Their great peculiarity, we have 

 already incidentally stated, is the want of union between 

 each other and between them and surrounding tissues. 

 They play an important part in the development of the 

 young thallus, and are the main element frequently in the 

 pulverulent form of the crustaceous thallus formerly alluded 

 to. They frequently burst through the cortical layer of a 

 Lichen, appearing on its surface in the form of powdery 

 masses, varying in size and colour, denominated soredia. 

 In the foliaceous thallus, on which they are commonly met 



Sciences Naturelles,' Second Series, 1840, and the 'Flora/ 1841 ; papers by 

 Thwaites and Dickie in c Annals of Natural History,' formerly the 6 Maga- 

 zine of Zoology and Botany,' vols. iii. and viii. : Cassini in his ' Opusc. 

 Phytol.' 1826; ' Botanical Gazette' (Miiller), April, 1849 (Thwaites), April, 

 1848, March, 1849, July, 1850. 



