Kelley — 138 — Mycotrophy 



do grow in union, and are thus found in association. The relationship 

 is probably not obligatory since the fungus, e.g., which grows on Finns 

 Taeda in America might grow equally well on P. sylvestris in Europe ; 

 but in actuality the two are separated by the Atlantic ocean and have 

 no means of coming together. In other words, the association is 

 obligatory only in the sense that the symbionts have no other means 

 of livelihood except in association. The humus-dwelling hymenomy- 

 cetes cannot live in arid mineral soil and cannot therefore associate 

 with dune grass or beach plum ; and it may be that some are limited 

 to certain specific compounds just as human individuals are limited 

 in metabolism to certain polypetides. At the same time it is perfectly 

 true that the otherwise "obligate fungi" can be grown in culture apart 

 from their usual symbionts ; yet the laboratory and the woodland are 

 two different things, and without being too cynical we may say that 

 our experience with natural woodlands is that they are notably deficient 

 in supplies of Erlenmeyer flasks and culture media.* 



The Ericads: — The ericads offer a much-mooted relationship. 

 At least three cases of obligate symbiosis have been recorded for these 

 plants: {1) Francke (1934), in a study of Monotropa Hypopitys, 

 found that a fungus-free protocorm was never seen which, he thought, 

 indicated an obligate mycotrophisni. Kamienski in 1884 had raised 

 the question whether the relationship were obligatory. (2) Paula 

 FiJRTH (1920) made the unconfirmed report of obligate symbiosis 

 in Pyrola in Lower Austria. (J) Rayner (1915) claimed to have 

 proved an obligate relationship in Calluna vulgaris. A fungus identi- 

 fied as Phyllophoma was found to grow not only in roots but through- 

 out the whole plant — stems, leaves, flower and seed-coat but not grow- 

 ing into the embryo. Infection of the developing plantlet was from 

 the seed-coats ; while sterile seedlings did not live long enough to 

 develop roots. But Christoph (1921) reported that Calluna in 

 nature is facultatively mycotrophic since the fungus is entirely absent 

 from habitats lacking humus, indicating that an obligate symbiosis 

 does not exist. 



The Problem of Calluna: — Calluna was further studied by 

 Knudson ( 1929) . He used seed grown in the U.S.A., and germinated 

 the seed on Rayner's solution A to which was added 1.5% of stand- 

 ardized agar. The seed was sterilized in a Ca hypochlorite filtrate, 

 usually for 30 min., and transferred by a looped platinum wire 



♦Systemic infection is also claimed by BosE for Casuarina. He says that 

 only the resting embryo is free. Seed infection was also found in tomato, apple 

 and several kinds of cereal grains (Nature 159:513-514, 1947). 



