LICHENS. 307 



soon begin to divide and subdivide again, and tiis process 

 may go on without much variation even for years. The 

 phenomena described may also be watched by taking a 

 portion of the bark of a tree on which the Chlorococcus 

 has been deposited, and placing it under a glass to keep it 

 in a moderately moist atmosphere ; the only difference 

 being a change in colour, which is caused by the growth of 

 the fibres, as may be seen on microscopical examination. 

 And this," Dr. Hicks says, "is an instructive point, be- 

 cause it will be found that the colour varies notably accord- 

 ing to the Lichen prevalent in its neighbourhood." x He 

 thinks there can be no doubt that what has been 

 called Chlorococcus, is nothing more than the gonidia of 

 some Lichen ; and that under suitable conditions, chiefly 

 drought and warmth, the gonidium often throws out from 

 its external envelope, a small fibre, which, adhering and 

 branching, ultimately encases it and forms a " soridium." 

 ' The soridia also remain dormant for a very long time, 

 and do not develop into thalli unless in a favourable 

 situation ; in some cases it may be for years. It will be 

 easily perceived that the soridium contains all the elements 

 of a thallus in miniature j in fact, a thallus does frequently 

 arise from one alone, yet, generally, the fibres of neigh- 

 bouring soridia interlace, and thus a thallus is matured 

 more rapidly. This is one of -the causes of the variation 

 of appearance, so common in many species of Lichens, and 

 is more readily seen towards the centre of the parent 

 thallus. "When the gonidia remain attached to the parent 

 thallus, the circumstances are, of course, generally very 

 favourable, and then they develop into secondary thalli, 

 attached more or less to the older one, which, in many 

 instances, decays beneath them. This process being con- 

 tinued year after year, gives an apparent thickness and 

 spongy appearance to the Lichen, and is the principal 

 cause of the various modifications in the external aspect 

 of the Lichens which caused them formerly to be mis- 

 classified." 2 



(1) "For instance, where the yellow Parmelia is found, the Chlorococcus will 

 assume a yellow tinge in its soridial stage. Viewed by transmitted ligbt, they 

 are also opaque balls, with irregular outline." 



(2) " Contributions to the Knowledge of the Development of the Gonidia of 

 Lichens." By J. Braxton Hicks, M.D. &c., Quarterly Journal of Microscopical 

 Science, vol. viii. 860, p. 239. j 



x2 



