792 



ECOLOGY 



outside), while if the fungi occur within the roots, as in the orchids 

 (fig. 1 1 06), it is called endotrophic {i.e. nourished within). Ecto trophic 

 mycorhizas vary from forms with loose and scattered hyphal threads 

 which come into casual contact with the roots to a condition like that in 

 Monoiropa, where the root system usually is compacted into a clump or 



ball, and where the in- 

 dividual rootlets are so 

 closely invested by fun- 

 gal hyphaethatthelatter 

 when sectioned resem- 

 ble a compact tissue 

 (figs. 1 104, 1105); in 

 such a case the root 

 proper does not come 

 into contact with the 

 soil. The hyphae com- 

 posing the fungal root 

 sheath connect with the 

 mycelia that permeate 

 the humus. The root- 

 lets of the beech and 

 of most plants with 

 prominent ectotrophic 

 mycorhizas are rela- 

 tively short and thick 

 and have a coralloid 

 aspect; moreover, 

 growth is relatively 

 sluggish and root hairs 

 are few or wanting except on roots or on parts of roots that are 

 comparatively free from fungi. 



Endotrophic mycorhizas, which especially characterize the orchids, 

 are in many respects much more specialized than are the ectotrophic 

 forms. Orchid roots are characteristically fleshy and tuber-like, differ- 

 ing much more from ordinary roots than do those associated with ecto- 

 trophic fungi; furthermore, the endophytic fungi appear to be specialized 

 forms rather than ordinary soil fungi. Certain cortical cells contain 

 closely interwoven clumps of hyphae which commonly enfold the nucleus, 

 and there are hyphal connections with similar clumps in adjoining cells, 



Figs. 1104, 1105. — Mycorhiza of the Indian pipe 

 {Monotropa uniflora): 1104, the basal portion of a. stem 

 with its dump of roots; note the coralloid root system (r), 

 and the imbricated scale leaves (s); 1105, a cross section 

 through the outer part of one of the coralloid roots, 

 showing the compact arrangement of the fungal hyphae 

 which iorm a pseudo-parenchyma (/) ; note that the fungus 

 is partly ectotrophic and partly endotrophic, the hyphae 

 penetrating into the epidermis (e) of the root and crowd- 

 ing aside the cytoplasm (c) of the latter; «, the nuclei of 

 the epidermal cells; 1105 highly magnified. 



