796 



ECOLOGY 



specialized fungi, such as Rhizoctonia lanuginosa or R. mucoroides, can institute 

 a long-lived symbiosis in but a few of the higher orchids; if other orchids are arti- 

 ficially infected with the latter fungi, one or the other of the symbionts soon dies, 

 since the parasitism is one-sided and highly detrimental rather than reciprocal and 

 moderate. The far-reaching influence of the fungal symbiont is well illustrated 

 by the fact that the structure and behavior of the orchid may vary if it is infected 

 by a new fungus; for example, Bletilla develops a corm, when it is infected by the 

 specialized fungus of Cattleya. The precise influence of the fungus upon germina- 

 tion is not known; it has been claimed recently that 

 it secretes diastase, which transforms the starch of 

 the seeds into sugar. The orchids are not alone 

 in requiring the presence of fungi at germination, 

 since the spores of Lycopodium are unable to de- 

 velop beyond a five-celled stage in cultures that are 

 free from fungi. 



The data just given appear to indicate 

 that some mycophytes, at any rate, are para- 

 sitic or at least dependent upon their root 

 fungi. There is evidence also of parasitism 

 in the opposite direction. Gall formation, 

 cell hypertrophy, and nuclear disorganiza- 

 tion have already been mentioned, and they 

 indicate the probable parasitism of root 

 fungi upon green plants; since these fungi 

 also live as saprophytes in the soil, they 

 appear to belong to the group of facultative 

 parasites. Careful cytological study in 

 Neottia has shown that in certain cells (fun- 

 gal host cells, fig. 1109) the hyphae have 

 haustoria and are vigorous and healthy, 



1110 



FIGS. 1109, 1 1 10. Cells 

 from the root of an orchid 

 (Ncotlia Nidus-avis): 1109, a 

 fungal host cell in which the 

 hyphae (h) are vigorous and 

 apparently parasitic upon the 

 orchid; n, nucleus; mo, a 

 digestive cell in which the hy- 

 phae are being disorganized 

 and digested by the orchid ; 

 highly magnified. After 

 MAGNDS. 



clearly living parasitically on the orchid, 

 while in other cells (digestive cells, fig. mo), the orchid appears to be 

 destroying and digesting the fungal hyphae. Similarly contrasting host 

 cells and digestive cells have been observed also outside of the orchids, 

 as in Podocarpus and Psilotum. These are representative instances 

 of reciprocal parasitism, and it is rather likely that similar nutritive 

 relations occur in many other mycorhiza plants, though how widely 

 it is not yet possible to say. 



The substances appropriated from one another by the two symbionts are 

 not certainly known, though there is strong presumptive evidence in 

 certain cases that the interrelations in part resemble those existing be- 



