Lecture X — 141 — Obligate Symbiosis 



BuRGEFF (1932), who is so well qualified to speak on the subject, 

 distinguished amongst the orchids, holosaprophytes (plants with 

 normal roots but of a very limited number), hemisaprophytes (in- 

 cluding only Hcllehorine amongst orchids), and root saprophytes (in 

 which the root alone has taken over the saprophytic function of 

 acquiring carbon-compounds). Root saprophytes, he said, seem less 

 adapted to mycotrophy than rhizome saprophytes : Gastrodia is an 

 extreme example of a root saprophyte. Saprophytism of the germinat- 

 ing plantlet, independence of the embryo of its nutrient tissue, the 

 smallness and great number of the seeds, must go together with fungal 

 infection and the antagonistic phenomena of phagocytosis. But de- 

 terminate of results of infection is quality of the fungus together 

 with the amount of its transmitted organic material. The fungi are 

 of different sorts and lead by steps from Rhizoctonia to the cellulose- 

 splitting Hymenomycetes and thence to the wood-fungi; and with 

 increase in amount of nutrient translocated by the fungus there is 

 development of saprophytism and increase in size of the saprophytic 

 organ. 



Germination of Orchid Seed: — Mention of the germinating 

 plantlet leads to a consideration of obligate endophytism of orchid 

 seed germination. NoiiL Bernard, the gifted French botantist who 

 did so much for our science during his brief life, first made known 

 the fungal symbiosis which exists with orchid seeds. It had long 

 been a mystery why seeds germinated with such difficulty, although 

 germination had been observed as early as 1804 by Salisbury, who 

 described it from OrcJiis Morio and Limodorum. Bernard (1899), 

 by a chance observation of germinating seeds of Neottia. a stalk of 

 which had been accidentally buried in soil, saw that fungi (which he 

 called "mycorrhizes") were associated, and jumped to the conclusion 

 "that the mycorrhizae are indispensable to the plant at the time of 

 its germination". This conclusion he then set himself to verify, with 

 brilliant results. In 1900 he said: "The known difficulty in germinat- 

 ing orchid seed is due to the fact that presence of a fungus, normally 

 present in orchid roots, is necessary for their development." In 1902 

 he stated: "In orchids the mycorrhizal fungus penetrates even into the 

 germinating seed and apparently they are able tO' germinate only when 

 infected." Next year he continued : "Thanks to the help of M. 

 Magne, I am able to recount observations on germination of Cattlyea 

 and Laelia. The seeds of these species and their hybrids are the most 

 easily germinated of their series. . . . these experiments show that 

 penetration of the fungus is a necessary and sufficient supplementary 

 condition for their germination." 



