liv THE BEGINNINGS OF LIFE. 



its external envelope a small fibre, which adhering and 

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

 At this stage the gonidium may continue also for an in- 

 definite period in a dormant condition, but, circumstances 

 favouring, segmentation of the gonidium goes on within the 

 soridium, while the branches of the fibre penetrate within 

 the divisions, till at last a young thallus is formed 2 . But a 

 check may occur during any of these stages, and yet vitality 

 be prolonged for a period of months and even years/ 

 Frequently the ' Chlorococcus ' stage continues for a very 

 long time, and the products resulting from it constitute a 

 green coating on walls, trees, and other external objects, on 

 which it may cover large surfaces. The continuance of this 

 algoid stage seems to be favoured by the occurrence of cool 

 moist weather ; and its tendency to persist in this stage, 

 taken in conjunction with the almost universal presence of 

 the gonidia of Lichens 3 in snow and rain water, may help 

 to account for the wide diffusion of this green algoid 

 coating. 



Although the Chlorococcus - gonidium and the Lichen- 

 gonidium usually undergo precisely similar segmentation- 

 changes till soridia appear and the real lichen-thallus is ulti- 

 mately produced, yet, in other cases, the soridia undergo 

 another remarkable set of changes, instead of passing on 

 directly to the development of a thallus. The changes about 

 to be described have been observed by Dr. Hicks principally 



1 See loc. cit, PL x. 



2 Dr. Hicks says : ' It will be easily perceived that the soridium con- 

 tains all the elements of a thallus in miniature ; in fact, a thallus does 

 frequently arise from one alone, yet, generally, the fibres of neighbouring 

 soridia interlace, and thus a thallus is matured more rapidly. This is 

 one of the causes of the variation in appearance so common in many 

 species of Lichens.' 



3 After a series of observations extending over many years, Dr. Hicks 

 is enabled to say, that ' they may be collected in comparatively great 

 numbers from snow and rain, particularly the former.' 



