Kelley — 100 — Mycotrophy 



a fungus which fills the mediocortex and grows forward with the 

 apex. The nucleus is seldom affected by hyphal invasion. Corallorhiza 

 innata of the Alps has a coralloid rhizome which bears papillae from 

 which tufts of hair arise, and at the tips of the hairs chemical changes 

 seem to take place. Hyphae pass directly from the soil through the 

 hair into the rhizome, going through the outermost layer of cells 

 (which are rich in starch) to a zone in which they coil within thin- 

 walled cells. There is a paucity of starch in this zone but within is a 

 third zone in which starch increases in quantity as hyphae become 

 less numerous, and all stages in degeneration of fungus may be seen 

 in cells of this region. The nucleus enlarges and contains bodies which 

 stain deeply with Hoffman's blue (Jennings & Hanna, 1898). 



In the Australian orchid, i?/M>aMf/t^//a, PiTTMAN (1929) described 

 fungal infection of the rootless rhizomes to a depth of ten cell layers, 

 the epidermis being fungus-free. Clots are illustrated as chiefly in 

 the exocortex. Infection was through hairs borne on the mycor- 

 rhizome. No arbuscles, vesicles or sporangioles were seen, but the 

 hyphal clots degenerate into a golden-brown mass. 



In Gastrodia the fungus inhabits superficial cells of the fleshy 

 succulent rhizome (McLuckie, 1923), the fungus being Armillaria 

 (KUSANO, 1911). 



Various other orchid mycorrhizomes have been described, as in 

 Orchis, Cephalanthera, etc. The general structure is always the same 

 however, as summarized by Burgeff (1909) : The mycorrhizal 

 fungus enters through hairs into the most external cells of the mycor- 

 rhizome and penetrates the cortex even to the endodermis, dissolving 

 whatever starch is present as it goes. Then the hypha coils within the 

 cell, and the cell plasm digests it, the undigested remainder being sur- 

 rounded by a membrane that excludes it from the living portion. A 

 few hyphae, in many species of orchid, grow out of the rhizome again 

 and form spores. In German orchids. Ad. Fuchs (1924) found di- 

 gestion occurring chiefly from autumn into winter. Penetration of the 

 fungus is accompanied by solution of the starch in the plant cells. The 

 fungus follows the concentration gradient and only in such places 

 as react to the fungal passage. The so-called protein hyphae contain 

 an evident preponderance of glycogen, and the designation of protein 

 in connection with them is an error. As a result of living in a region 

 poor in N and P, the orchid undergoes modification (Fuchs & 

 ZiEGENSPECK, 1925) : There is an early cessation of root-develop- 

 ment ; the rhizome swells and takes over the root function as the roots 

 dwindle and disappear. There is lessening of the water intake, a 

 crumpling of the habdrome while the leptome is kept open. 



In Thismia americana, Pfeiffer (1914) found underground struc- 



