332 



STRUCTURE AND LIFE HISTORIES 



branched, coralloid nodules on their roots (Fig. 241). 

 These roots are caused primarily by infection with the 



Fig. 239. — Thallus and erect portions (podetia) of two species of lichen 

 of the genus Cladonia. The larger podetium is proliferating, and bears a 

 number of apothecia on' the margin of the podetium of the first rank. 

 Apothecia are also shown on the margin of the larger apothecium at the 

 right. (Drawn from nature.) 



Fig. 240. — A lichen produced synthetically by cultivating the spores 

 of a fungus in association with a green alga. The culture was carried on 

 in a test-tube on sterilized culture media, thus preventing contamination 

 by bacteria and foreign spores. (After Bonnier.) 



nitrogen-fixing organism, Pscudomonas radicicola. By 

 the development of lenticels at the base of each nodule 



