GENERAL CHARACTERS OF BRITISH LICHENS. 71 



and inter twinings result in the formation of the hypothallus, 

 as we have already seen. For a considerable period the 

 characters of the spore otherwise continue the same ; gradu- 

 ally however its surface becomes shrivelled and opaque ; its 

 contents, whether cells or granules, disappear ; and finally, 

 the spore loses form, or is soon lost amid the hypothalline 

 network of filaments. Some authors have alluded to a pe- 

 culiar motion as occurring in the spores of certain species 

 when mixed with water ; but there is every reason to be- 

 lieve that this has merely been the molecular or Brownian 

 movement, now so familiar to botanists." 5 *" The mechanism 

 of the expulsion of the spores to the surface of the thalamium 

 is a very curious one : it appears that, under the influence 

 of moisture, the thalamium expands, while the exciple con- 

 tracts ; the result is a degree of pressure upon the thecse 

 sufficient to cause their rupture and the simultaneous ex- 

 pulsion of the spores. 



The Spermogones [airepfia, a seed, and yovr], generation) 

 are usually minute capsules or cysts, immersed in the sub- 



* It is designated molecular from its frequency in organic or inorganic 

 matter in a state of fine division when suspended in water ; and it is called 

 Brownian from its nature having been first distinctly pointed out by one of 

 our greatest living botanists, Robert Brown, of the British Museum. 



