HICKS, ON GONIDIA OF LICHENS. 19 



bably under the influence of the same moisture) elongate, be- 

 come more delicate, and as the soridium breaks up they become 

 detached, whilst their origin is rendered obscure. I have seen 

 them gradually become very delicate, and dipping between the 

 mucous covering of the Pahnoglaa-iovm cells in almost every 

 specimen. This, I think, clears up the mystery hanging over 

 these delicate fibres, which have been a source of much dis- 

 putation amongst some of our best observers in this branch. 

 If the reader will refer to my former contribution on this 

 subject, he will observe that it was remarked that the 

 '^ Chlorococcus ^' of any given neighbourhood varied very 

 constantly with the prevalent lichen of that spot, and this 

 remark applies peculiarly to localities where Cladonia prevails. 

 If any old wall where Cladonia is growing be observed care- 

 fully, it will be found that where the Chlorococcus has gone 

 on to the formation of soridia (provided the weather be damp 

 and moderately warm) that all the changes mentioned above 

 are taking place within the latter. It will be seen that, sooner 

 or later, over a considerable surface originally covered by Chlo- 

 rococcus, the latter has been supplanted by a P almella-f orm of 

 growth, forming broad patches of a gelatinous " frond," and 

 these growths proceeding rapidly, the stratum soon acquires 

 considerable thickness. By comparing and tracing the 

 formation of the Cladonia- gonidium, and its spreading away 

 from the parent lichen to form a Chlorococcus, and by 

 noting the subsequent changes it undergoes till it forms a 

 broad patch of a P a Imella- form, growth, I conceive it will 

 readily be conceded by any one taking the pains to observe 

 that the origin of the latter so-called alga is as above 

 described. 



What are the required changes of circumstances which tend 

 to direct cell-growth into this or that form of subdivision is 

 still inexplicable ; it suffices here to state a palpable con- 

 dition ; but whatever changes of form and appearance they 

 may undergo, I have no doubt, from numerous observations, 

 that even these Gleocapsa, &c., do, by the condensation or 

 desiccation of the mucous sheath and by the enlargement of 

 the green cell, ultimately revert to the form of the original 

 gonidium from which they arose. 



Perhaps the best example in support of the above remarks 

 is to be found on the 23odetia of Cladonia pyxidat a, where, by 

 watching from time to time the gonidia as they appear on the 

 surface, the whole process may be observed. It may also be 

 noticed at one and the same time on different parts of the 

 podetia ; for in the scyjjhus, or cup, are found the Chlorococcus 

 stage and soridia ; half-way down, the Gleocapsa stage; and 



