ENDOTROPHIC MYCORRHIZAS 109 



with assurance concerning many of these forms but at present it 

 seems Hkely that most ectotrophic mycorrhizas of dicotyledonous 

 trees represent examples of antagonistic nutritive conjunctive 

 symbiosis while those of coniferous trees may be reciprocal. In the 

 case of the Indian Pipe {Monotropa uniflora) the relationship is 

 almost certainly reciprocal. This little, colorless seed plant was 

 formerly thought to be a saprophyte. Usually, however, the entire 

 root system of the plant is transformed into a cluster of ectendo- 

 trophic mycorrhizas and it appears that the seed plant must be 

 parasitic on the fungus as well as the fungus being parasitic on the 

 seed plant. 



66. Endotrophic Mycorrhizas.— Endotrophic mycorrhizas differ 

 from ectotrophic in that the mycelium of the fungus occurs inside 

 the cells of the root rather than between the cells or on the outside of 

 the root. In the case of the red maple, Acer mhnim, the rootlets 

 containing the fungus are transformed into small bead-like galls 

 (Fig. 52). There may be a single one of these in the case of a short 

 rootlet or there may be several, arranged like a short string of beads, 

 on a longer rootlet. The fungus is found only in the cortical cells 

 of these bead-like mycorrhizas. Although these mycorrhizas of the 

 maple are usually quite abundant on the roots in the superficial 

 layers of the soil they are not thought to be of any great importance 

 to the tree. No experimental work has ever been done on them, 

 however, so that the exact relation between tree and fungus is not 

 known. 



Among the most interesting of endotrophic mycorrhizas are those 

 of the orchids. ]Many orchids are entirely dependent upon mycor- 

 rhizal fungi. The seeds of some orchids, under natural conditions, 

 will not germinate except in the presence of the mycelium of the 

 mycorrhizal fungus. In other species the seeds germinate but do not 

 develop beyond the seedling stage unless they become infected with 

 the proper kind of fungus. These facts have made the propagation 

 of orchids for commercial purposes very difficult, and in some cases 

 impossible, in the past. The mycorrhizal fungi concerned with 

 most of these orchids, as well as with other endotrophic mycorrhizas, 

 are not mushrooms but microscopic molds which for the most part 

 are not well known although a few have been identified. Gastrodia 

 elata, a non-chlorophyllous orchid of Japan, however, has as its 

 endophyte the mycelium of Armillaria mellea which is a common 

 edible mushroom. 



Experiments have shown that the seeds of many orchids can be 



