HICKS, ON GONIDIA OF LICHENS. 241 



in many, constant minute differences, visible to a practised 

 eye, while in some there are essential variations during their 

 growth, which will be noticed hereafter. 



After these remarks I will pass to the consideration of that 

 unicellular plant, commonly called " Chlorococcus," which 

 covers with a green coating, walls, trees, palings and indeed 

 any exposed body rough enough to give attachment to it. 



It is, in its mature, quiescent state, a round, globular cell 

 (fig. 1, a, a), consisting of a cell-wall, with green cell-contents, 

 having a nucleus in its centre. It is shown highly magnified 

 at fig. 3. These cells may remain in a dormant condition for a 

 considerable time during cold weather, but upon the return 

 of warmth and moisture they begin to increase in numbers, 

 by a process of subdivision which varies in the different 

 cells. 



Sometimes the mass of contents divides into from two to 

 eight or more portions, which soon assume a round form, and 

 burst the parent cell- wall open; or the septa radiate from the 

 centre ; these secondary cells soon begin to divide by binary 

 and quaternary division, and this process may go on for a 

 very long period, even for years, without much variation. 

 The size of these divisions varies according to the rapidity 

 with which the process of segmentation exceeds that of 

 individual cell-growth (PI. X, figs. 1, 2). Ultimately, how- 

 ever, they all assume the form and size of the parent round, 

 nucleated cell. 



Now, the gonidia of many of the lichens are precisely 

 similar, both in the mature, quiet state, as also in the active 

 process of multiplication, and are of the same size. This is 

 well seen in making a section of the thallus of any ordinary 

 lichen about to undergo what is called " soridiferous de- 

 generation — for instance, of Parmelia parietina. The gonidia, 

 increasing beneath the cortical layer by subdivision, at first 

 elevate in parts the layer above it, till at length they burst 

 through, and then at first appear of a green colour, continuing 

 the process of subdivision in a manner indistinguishable in 

 ever}'- respect from the " Chlorococcus " before described. 

 For this reason it has been suspected by some recent inves- 

 tigators that the latter is possibly derived from the gonidium 

 of the lichens. 



The additional facts I shall bring forward will, I conceive, 

 set the question affirmatively at rest. 



It will, therefore, be necessary to watch the true gonidium 

 a stage further, while still resting on the thallus through 

 which it has burst. After the process of segmentation has 

 been repeated an uncertain number of times, and the divisions 



