80 NOTES AND MEMORANDA. 



had degenerated into gonidia and had given rise to the formation of the 

 cephalodia, the author examined the history of the development of the 

 latter with the following results : — 



On the surface of the thallus of Peltigera apJithosa are found 

 verrucose ceiDhalodia, which, as they approach the edge of the lichen, 

 become smaller and at last appear to the naked eye like a grain of 

 dust. The smallest are the youngest. Amongst these excrescences is 

 often found a bluish coating, which consists exclusively of Nostoc in 

 different degrees of development ; they are rarely found mixed with other 

 algfe. By making transverse sections of the youngest portions of a 

 perfectly fresh lichen, it may be seen under a high magnifyinc; power 

 that its surface is covered with a great number of hairs, formed of one, 

 two, or three cells ; among these hairs are often found whole colonies 

 of Nostoc, some of which are entirely free, simply in contact with the 

 surface of the hairs, and easily separating from them under the 

 pressure of the covering glass ; others, on the contrary, are so closely 

 attached to the hairs, that it is only by very strong pressure that they 

 can be detached, and then only by removing the hair at the same time. 

 The hairs connected in this manner with the colonies, undergo a 

 division into numerous cells, and put out little branches which pene- 

 trate into the interior of the mucilage and wind about among the 

 filaments of the isolated Nostoc. This is the beginning of the forma- 

 tion of the cephalodia. In the same sections, or in others made on 

 older portions of the lichen, colonies of Nostoc are met with where the 

 interlacing by the hypha begins. We see distinctly that some of the 

 branches insinuate themselves into the interior of the mucilage, whilst 

 others only touch the surface and give rise to the cortical layer by 

 forming numerous lateral branches which adhere to one another. At 

 this period the cortical layer does not cover the whole surface of the 

 colony ; the mucilaginous substance, which has become darker, is seen 

 projecting here and there, and in the interior the cells of Nostoc 

 spread themselves, no longer arranged in the form of isolated filaments, 

 but united into a compact mass. If this preparation is broken up, 

 there will be seen, amongst the cells of Nostoc, filaments of hypha 

 which start from the cells of the cortical layer. In sections made on 

 an older portion of the lichen, colonies of Nostoc are found entirely 

 interlaced by hypha, where the cortex is formed of a continuous layer 

 of cells, arising from the ramification and their reciprocal adherence. 

 At the first glance such formations might be taken for the fructifica- 

 tions of Pyrenomycetes, if the history of their development, as well as 

 their anatomical structure, were not already known. 



In proportion as the invasion of the Nostoc by the hypha becomes 

 complete, the cells of the cortical layer of the lichen and the hyphfe 

 of the gonidial layer rise considerably in their growth, and gradually 

 form with the cephalodium a continuous tissue. The gonidia of the 

 lichen, which are found below the cephalodium, perish and disappear 

 gradually, being absorbed by the surrounding tissue ; moreover they 

 are no longer arranged in a continuous layer, but an intermittent one. 

 In its more advanced stage, the cephalodium increases considerably in 

 a direction parallel to the surface of the lichen and takes a lenticular 



