FROM DUST TO DUST. 55 



In the well-known instance of lichens, the algal cells are 

 nourished by the hyphal tissue of their fungal overgrowths, which 

 weave themselves round and enclose them, and supply nitrogen 

 and inorganic materials; whilst the algal cells, m turn, convert 

 the carbonic acid of the air into organic compounds, such as 

 starch, which the fungus makes use of The fructification in 

 lichens is produced by these fungal overgrowths ; and it is inter- 

 esting to observe that in the proximity of large towns they do not 

 attain to this stage, because the smoky air is inimical to the growth 

 of the fungi. No doubt many of us have observed the (so-called) 

 " fairy rings " on our hills and downs ; these are produced by the 

 better and more vigorous growth of the grass, owing to the excess 

 of nitrogen affon^^ed by the fungi, which composed the ring of the 

 previous year. 



■ According to the recent experiments of Schloesing and Lau- 

 rent,* various mosses and minute algse (Confervae, Oscillatorige, 

 etc.), which usually develop on the surface of the soil, also absorb 

 nitrogen from the air, which, after having been assimilated to the 

 soil, goes to feed the higher plants. Whether or no these lower 

 plant organisms take the nitrogen direct from the air, or are 

 indebted to bacteria for their supply,t we do not at present know, 

 but that the higher plants are dependent upon bacteria and the 

 lower fungi for their nitrogen is clearly established. There is every 

 reason to believe that nitrifying organisms supply the lower fungi, 

 and that these, in turn, associate with algae to form lichens, and 

 afford nitrogen to them. \ Thus we ascend by the gradual increase 



* Comptes Rendtis, 1 89 1, t. 113, p. TJ^J. 

 t See Nature, July 19, 1894, under Notes commencing "Frank and others." 



ij: Nature., July 19, 1894, p. 276. — " Frank, and afterwards Schloesing 

 and Laurent, showed that soil containing bacteria and algae can fix free nitrogen 

 in large quantities ; their experiments, however, did not decide whether algae 

 alone are capable of doing this. In order to answer this question Kossowitsch 

 has estimated the amount of nitrogen present in a nutritive soil before and 

 after the growth of pure cultures of two kinds of algae — Cystococcus and 

 Stichococcus. In neither case was any sensible increase of nitrogen detected ; 

 so that it appears that neither of these algae alone have the power of fixing free 

 nitrogen. Cystococcus, even when mixed with pure cultures of the bacteria 

 which enable the Leguminosie to assimilate free nitrogen, was found powerless 

 in this direction ; whereas a mixture of soil-bacteria and Cystococcus, which 



