110 NUTRITIVE CONJUNCTIVE SYMBIOSIS 



germinated, and the seedlings grown, without fungi by using a proper 

 culture medium containing a sugar or some other chemical which 

 supplies the necessary stimulus. This is the method now generally 

 employed by florists. 



Some orchids, as Corallorhiza and Epipogon, have no roots, the 

 underground parts consisting entirely of branched rhizomes. These 

 rhizomes always contain endophytic fungi, however, and the 

 symbiotic relation is undoubtedly the same as in the case of orchids 

 with roots. Such a combination of fungus and stem may be called a 

 mycopremna. 



Endotrophic mycorrhizas are nearly as common in the Ericacese 

 as in the Orchidacea^, and certain genera of this family, as Calluna, 

 Rhododendron, and Vaccinium, the blueberry, are more or less de- 

 pendent upon mycorrhizal fungi. These fungi can grow only in an 

 acid medium and the blueberries and other mycorrhizal plants of the 

 Ericacege, therefore, can flourish only in a soil that is kept acid by 

 some means. 



In Calluna it has been found that the mycelium of the fungus, 

 which belongs to the genus Phoma, is not only present in the roots 

 but throughout the aerial parts of the plant as well and is always pres- 

 ent in the seeds. The germination of these seeds seems to be as 

 dependent upon the presence of the fungus as is that of orchid seeds. 



Endotrophic mycorrhizas are abundant in the composite, gentian, 

 and legume families and probably in some others but nothing is 

 known of their significance in these families. The fungi in the mycor- 

 rhizas of these families appear to belong to the group known as 

 Phycomycetes but none of them have ever been isolated for identi- 

 fication. 



A great deal of experimental work has been done on endotrophic 

 mycorrhizas in the orchid and heath families and in many of these 

 it is known that the fungus is parasitic on the root cells but that the 

 root cells finally digest and absorb the hyphaj of the fungus. The 

 relationship is, therefore, reciprocal. Outside of these two families, 

 however, not much work has been done and it is not definitely known 

 whether the various endotrophic mycorrhizal relationships are 

 reciprocal or antagonistic. 



67. Galls.— A gall is a structurally modified plant part caused by a 

 symbiotic relation with some other organism. Usually the aft'ected 

 part is enlarged either through increase in size of the cells (hyper- 

 trophy) or increase in the number of cells (hyperplasy) or both. 

 In some cases, on the other hand, there is a decrease in size through 

 a reduction in cell size (atrophy) or a reduction in cell number 



