240 



D. T. MACDOUGAL 



These occurrences raise interest in the relative conditions which 

 must prevail in two plants so that one becomes parasitic on the 

 other. The earlier experimental studies of the author led him 



Fig. 1. Sahuaro {Carnegiea giganlea) growing on prehistoric human grave 

 near Continental, Arizona. Opuntia growing as a parasite in axis of lowermost 

 branch. 



to conclude that a necessary condition of parasitism wa.s a 

 higher osmotic concentration of the species which could become 

 parasitic. 



