HICKS, ON GONID1A OF LICHENS. 243 



process in trie undoubted gonidium of the lichen. If a 

 portion of the bark of a tree on which the Chlorococcus is 

 growing be placed under glass, so as to keep it in a 

 moderately moist atmosphere, the phenomenon may be 

 observed in all its changes. It may also be traced perfectly 

 in nature, and may be recognised by lighter-coloured patches, 

 appearing where " Chlorococcus " has been growing. That 

 the change of colour is caused by the growth of the fibres 

 may readily be seen on microscopical examination ; and this 

 point is instructive, because it will be found that the colour 

 varies notably according to the lichen prevalent in its neigh- 

 bourhood. Where the yellow Parmelia is found, the 

 " Chlorococcus " will assume a yellow tinge in its soridial 

 stage. Viewed by transmitted light, they are also opaque 

 balls, with irregular outline (fig. 7). 



But it must be clearly understood that every Chlorococcus 

 does not follow exactly this course, for I shall show marked 

 exceptions; but it obtains with the generality; and it is a 

 remarkable fact, that when " Chlorococcus " does vary, it is 

 in the neighbourhood of those lichens whose gonidia also 

 vary, and in precisely the same manner. 



That this " Chlorococcus " stage does continue for a long 

 period without showing any disposition to form soridia, con- 

 stantly multiplying till large surfaces are covered, and to some 

 depth, may be plainly observed; and this, taken with what 

 I have remarked before, will explain its almost universal 

 presence. This condition seems to be favoured by cool, moist 

 weather. The soridia also remain dormant for a very long 

 time, and do not develop into thalli unless in a favorable 

 situation; in some cases, I think, for years. It will be 

 easily perceived that the soridium contains all the elements 

 of a th alius in miniature ; 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 favorable, and 

 then they develop into secondary thalli, attached more or less 

 to the older one, which, in many instances, decays be- 

 neath them. This process being continued year after year, 

 gives an apparent thickness and spongy appearance to the 

 lichen, and is the principal cause of the various modifi- 

 cations in the external aspect of the lichens which caused 

 them formerly to be misclassified. 



