SAPKOPHYTISJI AND SYMBIOSIS 



333 



the epidermis is ruptured, thus affording entrance to 

 another nitrogen-fixer, A zotobacter, and also under favor- 

 able conditions, to a blue-green alga (Nostoc) . This is the 

 only known case in which four organisms are associated 



Fig. 241. — Root-nodules of Cycas revoluta. n, one of several cross- 

 sectional views, showing the zone of the syinbiont alga, Nostoc. 



together symbiotically. The alga has never been found 

 in the nodules of Bowenia, Ceratozamia, Macrozamia, nor 

 Zamia (all genera of c}-cads). 



309. Grafting. — One of the oldest practices of horti- 

 culture is that of grafting a twig or a bud of one species 



