Lecture X — 139 — Obligate Symbiosis 



directly to the culture tubes without rinsing. In the first experiment, 

 no effort was made to isolate seed from the floral tissues but the whole 

 mass was placed upon the agar. In every case an Alteniaria was found 

 contaminating the culture but owing to lack of sugar fungal growth 

 was slight. The roots of many of the plants were examined micro- 

 scopically and in no case was any fungus-root infection found. The 

 seedlings developed normally without fungal infection. In the second 

 experiment, because of contaminations in the preceding, sound seeds 

 were selected and the experiment was repeated. No one h.i.c. was 

 found most favourable for growth but availability of iron was limited 

 in more acid reactions. No root infection was found and the roots 

 developed normally. "The conclusion is inevitable from this and the 

 preceding experiment that the fungus is not essential for normal 

 germination of the seed." Knudson supposed that Rayner's unin- 

 fected seedlings had behaved "abnormally" either because of toxic 

 action of a possible excess of iron in the culture solution, or through 

 injury by mercuric bichloride in sterilisation. 



Rayner (1929) rebutted these arguments by stating first, that she 

 never used more than a trace of iron in solution (3-4 drops of 0.1% 

 solution per litre), and second that Knudson's sterilisation method 

 (with calcium hypochlorite) was inadequate and that he had there- 

 fore not destroyed the fungus in the seed-coats. 



Freisleben (1933), finding the problem still unsettled, made 

 culture experiments that demonstrated much better growth of Calluna 

 seedlings with fungus than without, thus confirming Rayner in respect 

 to benefit of mycotrophy ; but he also found it possible to grow sterile 

 seedlings, indicating the symbiosis is not obligate and that mycelium 

 does not penetrate seed. Indeed, he found much less general infection 

 of the plant than indicated by Rayner. Continuing his studies (1934) , 

 he found that endophytes of various Vaccineae could be exchanged, 

 indicating that there was no obligate symbiosis in these plants. 

 Lumiere (1919) had called attention to the fact that Stahl had 

 grown Vaccinium in sterile soil without difficulty. Molliard (1937) 

 studied Calluna still further and came to agreement with Knudson 

 and Christoph, that presence of mycorrhizae is not necessary to de- 

 velopment of heather. 



Other Ericaceae: — In Rhododendron, infection by the endophyte 

 is not an obligate condition of development of the higher plant, which 

 in fact can form roots and establish itself in the total absence of 

 micro-organisms (Gordon, 1937). Neither is there obligate symbiosis 

 in Azalea mollis: Melin (1921) experimented with this species and 

 concluded that mycorrhizal fungi are not found in the aerial parts. 



