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TEXTBOOK OF BOTANY 



called mycorhizal fungi. They may live wholly external to the root, merely 

 forming a sheath around it; the fungal filaments may grow into the cells 

 of the root; or the entire fungus may grow within the cells of the root 

 ( Fig. 225 ) . Such mycorhizal fungi are habitually present on the roots of 



Fig. 225. External and internal mycorhizal fungi. After Frank and Magnus. 



most green plants in forests. Some of them haye been reported as harmful 

 to certain trees, others as beneficial, or even essential to them. Under 

 natural conditions, pines (Fig. 226), heath plants, and beeches are ap- 



FiG. 226. Eastern white pine mycorhiza (A), and root (B) not infected by fungi. 

 Drawn from photo in Black Rock Forest Papers, 1:66, 1937. 



parently dependent upon mycorhizal fungi, for their seedlings live but a 

 few years if deprived of them under experimental conditions. 



Diversity among the non-green plants. There are all sorts of gradations 



