SAPROPHYTISM AND SYMBIOSIS 



793 



so that the mycelium often is continuous (figs. iio6, 1107); frequently 

 also haustoria are present, and sometimes the internal hyphae are con- 

 tinuous with the hyphae 

 which ramify the soil. 

 Root hairs commonly are 

 scarce and are more or 

 less filled with hyphae. 

 Many root cells are free 

 from fungi, including 

 some of the outer cells 

 as well as those of the 

 vascular tract.^ 



Endotrophic fungi are as- 

 sociated with the tuber-like 

 gametophytes of Lycopodium 

 and Botrychium (fig. 1108). 

 Transitional forms between 

 ectotrophic and endotrophic 

 mycorhizas are not rare, 

 being characteristic of the 

 Ericaceae ; in Monotropa, 



Figs. 1106, 1107. — Endotrophic mycorhiza of orchid 

 roots: 1 106, a cross section through a part of a root 

 of Aplectrum hyemale, showing dense clumps of fungal 

 hyphae (/) in some of the larger cortical cells, several 

 rows inside of the epidermis (e) ; f, root hairs ; v, vas- 

 cular tract; 1107, a single cortical cell from the root 

 of Spiranthes cernua, showing fungal strands (/), cyto- 

 plasm (c), and the nucleus («); 1106 considerably, and 

 1107 highly magnified. 



for example, hyphae from 

 the fungal sheath invade and modify the epidermal layer (fig. 11 05). Even ecto- 

 trophic fungi may penetrate into the root, though in that event they commonly 



are intercellular rather than intra- 

 cellular, as in most endotrophic 

 forms. In some pines there are 

 both ectotrophic and endotrophic 

 mycorhizas, the relative develop- 

 ment of the two kinds varying 

 with the habitat. In some climb- 

 ing plants (as Vanilla) the fungus 

 is both ectotrophic and endo- 

 trophic and is said to penetrate 



_ „ A .■ ii. u I . even the tissues of the supporting 



Fig. 1108. — A section through the gameto- r 



phyte of Botrychium, showing endotrophic plant as well as those of the liana, 

 mycorhiza, the fungi inhabiting the ventral There is little doubt that the 



region; considerably magnified. — After coralloid aspect of roots associated 



Jeffrey. with ectotrophic fungi and the tu- 



^ In Corallorhiza, which has no roots, fungi occur in the rhizome and in its "root 

 hairs," and in Aplectrum, which has ordinary endophytic root fungi, the removal of the 

 corm is followed by the development of coralloid rhizomes that bear "root hairs " and 

 contain fungi, precisely as in Corallorhiza, 



