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 
