SAPROPHYTISM AND SYMBIOSIS 



333 



the epidermis is ruptured, thus affording entrance to 

 another nitrogen-fixer, Azotobacter, 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 symbiont alga, Nostoc. 



together symbiotically. The alga has never been found 

 in the nodules of Bowenia, Ceratozamia, Macrozamia, nor 

 Zamia (all genera of cycads). 



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

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



