Kelley — 132 — Mycotrophy 



mixed on the same root. From the tubercle extend strands of hyphae 

 into the soil, as Kirchner said (1908) : "auch hier strahlen von der 

 Pilzscheide formliche Hyphen-Perriicken in den Boden aus, der zu 

 einer dichten Masse verflochten wird." Ordinarily these tubercles 

 live but one year but sometimes they are persistent and continue to 

 branch, forming a rhizothamnion. 



On P. mugho (Laing, 1923), conspicuous nodular bodies, often 

 over y2 inch diameter, are found on trees not more than 12-13 years 

 old. The nodules are sessile or stalked and frequently found detached 

 in the soil. Branching of the rootlets is considered checked by the 

 fungus which is purely ectotrophic and rarely penetrates cortical 

 cells. NoELLE (1910) also failed to find the intracellular infection 

 claimed by Kirchner ( 1908) . In P. sylvestris nodules were described 

 by Laing (1923) with endophytic infection, while Melin (1922) 

 also described them, stating that they are as large as a pea. Nodulous 

 roots on P. Cenihra were described by von Tubeuf (1888), who 

 quoted from Reess (1887), who seems also to have described them. 

 Nodules of P. montana are mentioned by Somerville (1911). 

 Muller (1902) thought that they are of service in fixation of at- 

 mospheric nitrogen. 



Outer Cortex and Passage Cells: — In plants that develop a 

 thickened outer layer of cortex, or exodermis, in the rootlet, there are 

 left some thin-walled cells in the layer that do' not develop the wall- 

 thickening; and these thin-walled exodermal cells form a con- 

 venient means of access to the cortex beneath for the invading hyphae. 

 When so used by the endophyte these thin-walled cells are termed 

 passage-cells. They were described by Janse from Spadiciflorae and 

 named "cellules de passage", and he said that they are found in all 

 orchids. Burgeff called them "Durchlasszellen" (Demeter, 1923). 

 Passage-cells were described from Tipularia by Clifford (1899), 

 from Aphyllorchis by Groom (1894), from Dipodium by McLuckie 

 (1922), from Vanilla by Cordemoy (1904), from Vinca and As- 

 clepias Cornuti by Demeter (1923), who found passage-cells also 

 in the endodermis, from Gentiana by Schimmler (1937) ; and they 

 are figured for Hoja carnosa by Busich (1913), who considered 

 the presence of passage-cells as an inherited character of the Asclepi- 

 daceae that aids mycotrophy and adapts the plants to their habitat. 



Wall Tubules : — A hypha, passing through a living cell-wall, may 

 stimulate the cell to form a growth about the hypha which has been 

 called a wall-tubule or, in German, a "Rohrentiipfel". Jeffrey 

 (1898) had observed in Botrychium mycothalli that a thick sheath 



