Lecture VII — 99 — Mycothalli and Mycorrhizomes 



sisting of plant and fungal material. Of the fungus-inhabited layers, 

 the digestion cells take the outer and inner while the host-cells take 

 the middle. The digestion cells are defined by Magnus as follows : 

 "If, in Neottia, the fungus in a cell does not take on the modification 

 which characterizes the host-cell but branches again after the 

 'meristem condition' into thin-walled hyphae that inevitably encounter 

 a certain developmental process, — death, robbery of content and final 

 mantling into a clot, a development not less sharply delimited than in 

 the host-cell, — we shall designate such a cell a 'digestion-cell' (Ver- 

 dauungszelle)" (p. 223). Infested cortical cells are enlarged, and 

 later formed cells are also enlarged, causing a change in the whole 

 structure. The plasm continually surrounds the fungus in the diges- 

 tion cell and upon death of the fungus a copious formation of vacuoles 

 takes place. Vacuoles neighbouring the wall-layer which is free from 

 the fungus unite to form a large sap-vacuole and thereby separate 

 the clot, which either remains suspended in the sap-vacuole or is com- 

 pletely separated from the protoplast by formation of a new internally 

 lying plasm-layer. Plasm of the fungus-inhabited cell never dies before 

 death of the whole root. Plasm segregated in the clot becomes 

 changed into a cellulose sort of a substance. Upon migration of the 

 fungus there arises a fine-grained starch which soon dwindles but 

 after death of the fungus reappears in a modified form. The nucleus 

 becomes constricted or amoeboid and intensely chromatophilic, but 

 after phagocytosis is completed the nuclei return to their former 

 barrel-shape. 



Bernard (1899) described the mycorrhizome of Neottia as ex- 

 hibiting three zones of cells: (i) a starch layer; (2) several layers of 

 cells filled with intertwined mycelial filaments ; (5) epidermis, without 

 starch or hyphae. Spiranthes autumnalis dififers from other Neottiae 

 (according to Beau, 1913) in being annual, but it has mycorrhizomes 

 which are the organs of reserve and at time of flowering of the orchid 

 are invaded by an endophytic mycelium as evidenced by a pronounced 

 yellow colour given the sections through bodies resulting from diges- 

 tion of mycelial coils. Towards the end of the flowering season new 

 "roots" are formed which must be infected from the soil. 



Orchids other than Neottiae have received attention : Calypso has 

 a coralloid-branched mycorrhizome but Lundstrom (1889) failed to 

 find infection in plants of C. borealis collected in southern Sweden. 

 But in C. hidhosa, MacDougal (1899) found fungus living in outer 

 cortex but not passing out through nodal trichomes; its hyphae are 

 septate and form vesicles. Inner cortex and apex are free from infec- 

 tion. Corallorhiza arisonica, according to the same author, has the 

 coralloid rhizome represented by papillae which are infested early by 



