Mycotrophy — ix— Preface 



(Prat, 1945) treats of gradients in mycotrophic plant tissues. 

 Resistance to parasites is an important function in plants. In myco- 

 trophy the gradient of resistance to the mycelium is slowly progres- 

 sive. Mycelium penetrating toward the apex is always checked, for 

 the axial gradient of resistance is directed toward the point of the 

 root and is progressive ; but the radial gradient of resistance is 

 abruptly corrected at the level of the endodermis. Chemical barriers 

 are more efficacious in developing resistance than physiological 

 barriers. The gradients vary according to season. Still other papers 

 (Magrou, 1944, 1946) deal with tuberisation and the factors which 

 control it. 



Acclimatisation has been proved aided by symbiosis. At Angers 

 (Blaringhem, 1937), building a well-aerated humus layer favourable 

 to growth of symbiotic fungi aided the acclimating of 400 spp. of 

 conifers and 150 forms of oak, which proceeded to make 3 to 5 

 times the growth of the finest specimens of these species in their 

 native haunts. 



Two general papers on mycotrophic symbiosis have appeared re- 

 cently. In one (Owen, 1947), symbiosis is examined with reference 

 to the true character of the symbionts. It is concluded that true 

 mutualism exists between nodule bacteria and legumes, and perhaps 

 with mycorrhizae. But orchids are considered dependent on fungi, 

 since the fungi can live apart saprophytically. With characteristic 

 thoroughness Burgeff (1943) has analyzed mycotrophic phenomena 

 and presented a new classification of them. Realizing that their most 

 important function is material exchange which is dependent on the 

 union of tissues of both components, he classifies "mycorrhizae" as : 

 (1) Tolypophagous, in which rhizoctonial fungi form pelotons that 

 are digested with release of fat, glycogen and nitrogenous material 

 into the orchid plant that harbours them. (2) Thamniscophagous, in 

 which arbuscles are digested, leaving sporangioles as excreta. Very 

 widely distributed in green plants, including ferns and liverworts. 

 In certain ferns and colorless saprophytes the arbuscles go over to 

 the preceding form and are hence termed thamniscotolypophagous{ !) 

 The fungi appear to be Endogonaceae, hence the mycorrhizae are 

 called phycomycetoid. (3) Ptyophagous. Found in such plants as 

 Gastrodia and Monotropa, these "mycorrhizae" show resorption of 

 materials released into cell by fungal hyphae. Free fungal bodies 

 called "phytosomes" are formed which, being resorbed, leave 

 "Exkretkorper". (4) Chylophagoiis. Sap resorption occurs in sub- 

 terranean colorless and saprophytic prothallia of Lycopodium. There 

 is no digestion and there are no excretion bodies. Sap is exuded by 

 guttation of hyphae into intercellular spaces. (5) Halmophagous. 



