242 HICKS, ON GOXIDIA OF LICHENS. 



have again become full-sized and globular, it begins to make 

 the first step towards the formation of the felted fibres of its 

 parent, and it will be observed that a small, colourless, tubular 

 projection appears at one spot on the surface of the cell-wall 

 (fig. 2 a), which, increasing in length becomes a tubular 

 fibre, which, whilst adhering closely to the exterior of the 

 cell, and articulated and branching (fig. 2, b, b, b), at last 

 completely encloses it by its ramifications, which vary in 

 colour. In the case of Parmelia purietina they are yellow, 

 and the round mass, opaque by transmitted light, and rough 

 on its outside in consequence of the branches of the fibres 

 not closely adhering by their ends, is denominated a 

 " soridium." This gives the powdery appearance to the 

 surface of the lichen upon which it rests, and has a consider- 

 able influence on the general colour of the plant, which thus 

 depends on the amount and colour of these enclosing fibres. 



The soridium may remain in this stage for an indefinite period 

 — for months, and, I suspect, even for years — in which event 

 the case produced by the branching, adhering fibres becomes 

 thickened and denser, as shown at fig. 9. This is generally 

 more apparent during the colder months, and is probably a 

 means of protection. 



When segmentation commences within this soridium ,'\t often 

 results in a very large number of subdivisions, as at fig. 10, a, 

 and the fibres, passing inwards bctAvecn the segments, separate 

 them, while the whole ball enlarges, so as to produce the first 

 commencement of a thallus. This point is of importance, as 

 will be remarked upon Avhen avc come to speak of the corres- 

 ponding stage in the "soridium" of Cladonia. But segmen- 

 tation of the enclosed gonidium may proceed simultaneously 

 Avith the fibre-growth, in which case the soridia assume the 

 appearance shown at figs. 1 and 5. Frequently the subdivisions 

 become oval and small, undergoing binary segmentation. 

 Fig. 6 represents an instance of this, in which the parent 

 cell-Avail is still seen partly dissolved. Fig. 8, «, also shows 

 this form. In others they arc globular from the commence- 

 ment, and simply increase in size, as at figs. 4, 5, 10, b, b, till 

 they are as large as the parent cell. 



The contents of one of these broken soridia is shown at 

 fig. .X, with fibres branching among the segments. 



Precisely the same changes take place in the so-called 

 " Chlorococcus." As 1 said before, the multiplication by 

 division may proceed during an indefinite period ; however, 

 circumstances favouring the tendency to form the fibre com- 

 mences, and a "soridium" is the result. To describe these 

 changes Avould be but to repeat the above remarks on the 



