HICKS, ON GONIDIA OF LICHENS. 91 



tion as the so-called Chlorococcus ; but generally they are 

 small and delicate in appearance (fig. 1 a). For hov/ long a 

 period this process may proceed I am not in a position to show, 

 except that I have not met with any considerable mass 

 of Cldorococcus from this source. No doubt in most 

 instances they begin to assume a change which is analogous 

 to Cladonia-Gleoccipsa, for we find the results of segmenta- 

 tion become included in a common mucous envelope, such as 

 Gleocapsa possesses. But there are these differences at first; 

 that while in Cladonia-Gleocapsa the mucous layer is colour- 

 less and delicate, that in Collema is more dense and solid, and 

 coloured more or less of a bluish-green hue. Also, while in 

 the former growth the results of segmentation are nearly 

 always symmetrical, in Collema they are generally irregular, the 

 outline of the protoplasm being indistinct at the commence- 

 ment of these changes. This is shown at figs. 1 6, 2 a, 8 a. 



After these early stages the growth proceeds on various 

 plans. In one method the whole mass, both cell-contents and 

 the mucous layer, are of a dark purple colour ; each cell 

 undergoing binary division is surrounded by its own mucous 

 layer, the whole being included in a common one (fig. 2 b). 

 After a while the purple coating becomes colourless and fused 

 into one ; while the cell-contents become green, and the 

 divisions separated (fig. 3) . As segmentation proceeds, the re- 

 sulting cells assume a linear tendency, till at last a number of 

 moniliform filaments are formed, having here and there the 

 vesicular cells {heterocysts) found in the Nostochacepe (fig. 4). 

 Thus Collema passes into Nostoc. A second manner is repre- 

 sented at fig. 8, At ci, the early changes above alluded to are 

 shown ; the protoplasm and the mucous coating becoming of 

 a bluish-green hue ; but after the segmentary process has 

 proceeded a little, the latter becomes more transparent, colour- 

 less, and highly refractive (fig. 8Z»). Sooner or later there is 

 a disposition shown in the subdivisions of the cells to arrange 

 themselves linearly, whereby a small Nostoc-mass is pro- 

 duced; the vesicular cells appearing about the same time (fig. 

 8, c, d). Thus again Collema passes to Nostoc. 



But the connection between the two is more clearly shown 

 in tlie fact, that the gonidia of Nostoc pass through precisely 

 the same changes; so that a description of them would be super- 

 fluous, being a repetition of the facts just stated. 



iNIany variations occur during the formation of the above 

 into the mature Nostoc. 



For instance, when the development has arrived at the stage 

 indicated at fig. 8 c, and the vesicular cells are just forming, 

 the mass becomes converted, in part or entirely, into the 



